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Yellow light healthier during night shifts People

05 December 2013

People who work in shifts for long periods run health risks. The probable cause is a disruption of their circadian rhythm, which is controlled by the biological clock (a group of cells located in the brain). Chronobiologist Maan van de Werken from the University of Groningen discovered that working at night in yellow light causes the fewest physiological problems. She also discovered that the stress hormone, cortisol, reacts most strongly to stimuli early in the morning.

Night work is becoming increasingly common in our 24-hour economy. But people who spend long periods working shifts have a higher risk of diseases such as cancer, diabetes, obesity or depression. ‘It is therefore important to work out exactly what is causing these risks’, says Van de Werken.

Artificial lighting is a factor known to affect the way our body reacts to working at night. ‘The blue component in white light keeps people awake better than yellow light’, explains Van de Werken. For her study, she asked test subjects to stay awake for one night in yellow, white or half-light.


‘We saw that white light strongly suppresses the hormone melatonin.’ The body only produces melatonin at night, which is why it has been dubbed the ‘sleep hormone’. Incorrectly, according to Van de Werken: ‘I found no link whatsoever between tiredness and melatonin concentration.’

Interestingly, she did identify a link with skin temperature. Under normal circumstances, skin temperature rises at night. In white light, however, the temperature of the skin was considerably lower than in yellow light. Yellow light also prevented the drop in melatonin concentration. ‘My conclusion is that the bodies of the test subjects working in yellow light showed a more normal night-time pattern than of those working in white light.’

So yellow light is healthier because it interferes less with the body’s physical night-time reactions than white light. ‘I would definitely advise using yellow light during night shifts’, says Van de Werken. There is one important exception: ‘The test subjects working in yellow light felt slightly less alert, which corresponds with the higher skin temperature we measured. People with night-time jobs requiring a high degree of alertness, such as air traffic controllers, should not work in yellow light.’

Stress hormone

The concentration of the stress hormone cortisol rises from about 4 a.m. onwards. It peaks around half an hour after a person wakes up, informing the body that its active period has started. However, Van de Werken discovered that during this phase of the biological clock, the body is more sensitive to stimuli, which in turn causes a further rise in the level of cortisol. ‘There is a multiplier effect. If you have something to eat at the end of a night shift, or do something else that causes stress, you see an extreme rise in the concentration of cortisol.’

A long-term high concentration of cortisol can lead to the same problems as those caused by shift working. So cortisol deregulation could be another reason for extra health risks for people working shifts. ‘It would therefore be better to avoid things that raise the cortisol level early in the morning.’

Skin cells

Van de Werken found strong individual differences in the extent to which light suppresses melatonin. ‘Some people respond with a sharp drop in melatonin. You can measure this in their sputum. These people also demonstrated a different level of molecular reaction to a stimulus in their skin cells.’

A sharp drop in melatonin can be a sign that a person is susceptible to developing health problems during shift work. ‘So it may eventually be possible to tell whether someone is suitable for shift work by testing their skin cells.’

Wake Up Light

Van de Werken also tested the famous Philips ‘Wake-up Light’, a device that gradually increases the light in the room half an hour before the alarm goes off. ‘The skin temperature of the test subjects showed that they were completely awake sooner than without the light.’


Maan van de Werken, who conducted her research in the Department of Chronobiology, will be awarded a PhD by the Faculty of Mathematics and Natural Sciences at 2.30 p.m. on 6 December 2013. The title of her thesis is Dawn, light at night and the clock. Impact on human alertness, performance and physiologie.

The ceremony will be immediately followed by that of her husband, Mirre Simons, who will be awarded a PhD for research into sexual signalling and ageing among animals, including the zebra finch and the stickleback. Both now live and work in Sheffield.

Last modified:13 March 2020 02.18 a.m.
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