Why artificial daylight could brighten up your day

Tired and depressed? The lights at work may be to blame. Is artificial daylight the answer?

Everyone has experienced the post-lunch dip - the irresistible desire to snooze for 10 minutes after a meal. If you have deadlines to meet, this is not good news. If other people's safety depends on you - air traffic controllers, for example - the consequences could be disastrous.

Scientists have long been interested in the biological rhythms that affect our levels of alertness during the day. They know that the post-lunch dip occurs even in people who don't eat lunch. Now they believe these rhythms can be influenced by a hitherto neglected feature of the office environment - the lighting.

The average office is lit by overhead fluorescent tubes with a brightness rating of about 500 lux. Researchers are now experimenting with lamps that emit a different spectrum of light - of a cooler, bluish hue. They believe that it may stimulate a newly discovered receptor in the eye, which some call the "third eye", which is thought to control our biological clock.

There are two kinds of photoreceptor in the eye that allow us to see - the cones, which distinguish colour, and the rods, which permit vision (in black and white) at night. The new receptor responds to light in a different, non-visual way, sending signals to the hypothalamus in the brain, which regulates circadian rhythms and the production of the hormone melatonin. The discovery, reported in the science journal Nature, stimulated lighting designers to experiment with light of different wavelengths.

Among the first people to be tested were workers in a call centre in Cheshire, where new lamps were installed on one floor of their nine-floor building. The call centre, dealing with claims from individuals with private health insurance, was considered a high-stress workplace.

An ordinary 60-watt bulb emits light with a colour rating of around 2,700 kelvin, indicating it lies towards the red/orange end of the spectrum. A standard fluorescent tube as used in offices nationwide has a rating of 4,000 kelvin. The new Activiva lamps, made by Philips, which look the same as fluorescent tubes, but, at £5, cost around twice the price, emit light of 17,000 kelvin. "It looks very cool, very close to daylight. What we found after a year was a clear perception among the staff that they felt more alert and were doing their work to a higher standard, with a reduction in the number of calls not dealt with effectively. This represented a significant difference from the other floors," said Alan Clough, a product manager at Philips. The research is still in its infancy and scientists do not fully understand the effects of light in controlling biological systems. But there is a new recognition that lighting designed for offices and homes, which up to now has focused solely on its visual aspects, will have to change.

Professor George Brainard, the director of the light research programme at Jefferson Medical College in the US, whose work led to the development of the Activiva lamps, said: "Every built environment will ultimately change to accommodate the effects of light on biological systems. Lighting engineers have worked on providing light that is the best for seeing, comfortable to work under and looks nice. But that doesn't serve our biological needs. At certain times of the day we may want light that is richer in reds, and at other times richer in blues."

Experiments run in Professor Brainard's laboratory have shown that the sensitivity of the new receptors peaks with light towards the blue end of the spectrum - of a shorter wavelength than normal artificial light. The results, published in The Journal of Neuroscience, indicate that the eye has two different sensory systems - visual and biological - that are tuned to different parts of the spectrum.

Professor Brainard said: "The transmission [from the new receptors] peaks in the wavelength range of 446-477 nanometres for non-visual response. [Light] of around 555 nanometres [is] accepted as the most efficient level of light for daytime vision. It is striking that the 446-477 nanometre range of light is blue, of the same quality as in a clear daylight sky. Daylight has an abundance of these wavelengths, as well as those in the 555 nanometre range, satisfying both perceptual and biological demands. The task at hand, therefore, is to find lighting solutions which will perform as daylight."

The hope is that manipulating lighting may improve health, and tackle problems such as depression, sleep disturbance and daytime variation in levels of alertness. Previous research has shown that exposure to light can help shift-workers adjust their circadian rhythms, and sufferers beat winter depression.

"In the biology of humans there is never one hormone that controls all this. It is part of a complex system. We don't know if melatonin is the cause of winter depression. It appears to be involved but we are not sure how. The discovery of the new receptor, separate from the visual system, has opened the door to the idea that light could be used in this way," Professor Brainard said.

Studies at the University of Surrey's sleep research centre, have shown that people deprived of sleep function better when exposed to bright light. Professor Derk-Jan Dijk, the director of the centre, said: "If you have had little sleep, the more intense the lighting, the better your productivity will be. Our study showed that if you have bright light of 1,000 lux, it is better than dim light.

"The discovery of the 'third eye' is evidence that light does more than affect vision - it has an effect on the emotions. Lighting has always been designed so that we can see well, but in future we will have to look at its spectral composition, too."

What light can do for us

* Alleviates seasonal affective disorder (SAD)

* Increases the length and quality of sleep

* Improves the performance of night-shift workers

* Improves weight-gain in premature infants

* Activates the circadian system via a newly discovered photoreceptor in the eye

* Regulates melatonin production, which affects the sleep/wake cycle