Ever wondered why working night shifts feels so brutal? Czeisler's... Show more
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6 Dec 2025
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Psychology - Examining the Czeisler et al (1990) Sleep Study with Shift Workers
Tyra-Angel
@xt_ang_studyx
1 / 3
The Light-Dark Experiment
Your circadian rhythm naturally syncs with daylight hours, making night shifts feel like you're fighting against biology itself. Czeisler wanted to see if reversing light exposure could reset workers' body clocks by up to 12 hours.
The researchers recruited eight healthy men aged 22-29 who'd never regularly worked nights. They controlled for everything that might mess with sleep patterns - no coffee, alcohol, or drugs during the study (checked through urine samples!).
The experiment had two phases: a baseline week of normal sleep at home, followed by a week of simulated night shifts in the lab. Half the participants worked under normal dim lighting (150 lumens), while the treatment group was blasted with incredibly bright lights (12,000 lumens) from 00:15 to 07:45.
Quick Fact: The bright lights used were about 80 times brighter than normal indoor lighting - imagine working under stadium floodlights!
Every hour during their shifts, participants completed cognitive tests and rated their alertness every 20 minutes. This gave researchers precise data on how their mental performance changed throughout the night.
Results That Changed Everything
The treatment group had to stay in complete darkness from 9am-5pm each day, with blackout curtains blocking all sunlight. The control group just used normal curtains and slept whenever they felt like it.
The results were dramatic. Treatment participants slept an average of two hours longer each day and their body temperature patterns shifted forward by nine hours - basically flipping their biological clock. The control group's rhythms barely budged.
Most importantly, the light therapy group showed massive improvements in both alertness and cognitive performance during night hours. Their bodies had genuinely adapted to working at night instead of fighting it.
Study Insight: The human body clock can shift by up to 12 hours in just 3-4 days with properly timed light exposure - much faster than anyone expected!
Czeisler concluded that circadian maladjustment to night work isn't inevitable. With the right light-dark treatment, workers could maintain better alertness, performance, and sleep quality during shift work.
Strengths, Weaknesses, and Real-World Impact
The study's biggest strength was using multiple biological measures (hormones, body temperature) alongside performance tests. The nine-hour shift in the treatment group was so large it definitely wasn't just chance.
However, the sample size was tiny and only included young men, making it harder to generalise to women or older workers. The lab-based night work (sitting at a desk) might not reflect the reality of many night jobs either.
The researchers couldn't control everything - participants went home during the day and might have been exposed to different foods, lights, or other zeitgebers that could affect their body clocks.
Real-World Application: These findings directly help night shift workers, from hospital staff to factory workers, potentially reducing accidents and improving health outcomes.
This research has huge practical value for improving worker safety and productivity. Many industries now use light therapy principles to help employees adjust to shift patterns more effectively.
The study supports our understanding of how light suppresses melatonin and promotes alertness, showing that biological and cognitive approaches to sleep research complement each other perfectly.
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Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
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Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
Psychology - Examining the Czeisler et al (1990) Sleep Study with Shift Workers
Tyra-Angel
@xt_ang_studyx
Ever wondered why working night shifts feels so brutal? Czeisler's 1990 study explored whether clever light therapy could trick your body clock into adjusting to night work, potentially solving one of modern work life's biggest challenges.
The Light-Dark Experiment
Your circadian rhythm naturally syncs with daylight hours, making night shifts feel like you're fighting against biology itself. Czeisler wanted to see if reversing light exposure could reset workers' body clocks by up to 12 hours.
The researchers recruited eight healthy men aged 22-29 who'd never regularly worked nights. They controlled for everything that might mess with sleep patterns - no coffee, alcohol, or drugs during the study (checked through urine samples!).
The experiment had two phases: a baseline week of normal sleep at home, followed by a week of simulated night shifts in the lab. Half the participants worked under normal dim lighting (150 lumens), while the treatment group was blasted with incredibly bright lights (12,000 lumens) from 00:15 to 07:45.
Quick Fact: The bright lights used were about 80 times brighter than normal indoor lighting - imagine working under stadium floodlights!
Every hour during their shifts, participants completed cognitive tests and rated their alertness every 20 minutes. This gave researchers precise data on how their mental performance changed throughout the night.
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Results That Changed Everything
The treatment group had to stay in complete darkness from 9am-5pm each day, with blackout curtains blocking all sunlight. The control group just used normal curtains and slept whenever they felt like it.
The results were dramatic. Treatment participants slept an average of two hours longer each day and their body temperature patterns shifted forward by nine hours - basically flipping their biological clock. The control group's rhythms barely budged.
Most importantly, the light therapy group showed massive improvements in both alertness and cognitive performance during night hours. Their bodies had genuinely adapted to working at night instead of fighting it.
Study Insight: The human body clock can shift by up to 12 hours in just 3-4 days with properly timed light exposure - much faster than anyone expected!
Czeisler concluded that circadian maladjustment to night work isn't inevitable. With the right light-dark treatment, workers could maintain better alertness, performance, and sleep quality during shift work.
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Strengths, Weaknesses, and Real-World Impact
The study's biggest strength was using multiple biological measures (hormones, body temperature) alongside performance tests. The nine-hour shift in the treatment group was so large it definitely wasn't just chance.
However, the sample size was tiny and only included young men, making it harder to generalise to women or older workers. The lab-based night work (sitting at a desk) might not reflect the reality of many night jobs either.
The researchers couldn't control everything - participants went home during the day and might have been exposed to different foods, lights, or other zeitgebers that could affect their body clocks.
Real-World Application: These findings directly help night shift workers, from hospital staff to factory workers, potentially reducing accidents and improving health outcomes.
This research has huge practical value for improving worker safety and productivity. Many industries now use light therapy principles to help employees adjust to shift patterns more effectively.
The study supports our understanding of how light suppresses melatonin and promotes alertness, showing that biological and cognitive approaches to sleep research complement each other perfectly.
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
Where can I download the Knowunity app?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
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Most popular content: Circadian Rhythm
Most popular content in Psychology
Most popular content
English - inspector calls quotes and analysis
Quotes from every main character
English Literature10
Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.9/5
App Store
4.8/5
Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user