THE REASON WINTER COMMUTERS GET SO TIRED

IN ADDITION to the colour and timing of light, the intensity of it is critical to the timing of our body clock (see main copy below).

And the problem is that many of us are not getting enough light to set our body clock. Compared with natural outside light, even in winter, artificial light in offices and factories is very dim.

Overall, natural light is some 50 to 250 times brighter than most artificial light we encounter.

Light is measured in lux. Natural light is around 10,000 to 100,000 lux, depending on the weather, compared with the 300 to 400 lux in the workplace (or 800 to 900 lux you would encounter close to a 60 watt bulb at home).

That’s why, no matter how many years workers spend on permanent night shifts, nearly all find their clocks don’t adapt to the demands of working at night. After leaving work in the morning, the night-shift worker will move from the dim light in the workplace to bright light outside, and their clocks lock on to the brighter light signal, setting them to daytime.

People who don’t experience much natural light at all, such as nursing home residents or commuters (during the short days of winter), have a different problem: they don’t get enough light to set the clock properly and, just like the blind veteran Mark Threadgold (see previous page), their body clocks drift — and with them, the times when they sleep and wake.

Most of them will want to go to bed later and get up later.

There will also be times when they find it difficult to sleep at night, while wanting to sleep during the day.

CHEAT WITH A ‘LIGHT BOX’

TO FIND out how bright (or dim) your workplace or home is compared with natural light, you can buy a lux meter for around £20 online. You could then even buy bright artificial lights to help set your body clock in the morning and see if this makes a difference to the times you sleep and wake.

One option is a light box, which simulates the dawn and dusk cycle (slowly brightening then fading) to help stabilise your body clock.

The main thing to look for is that they produce enough bright light, in the region of 2,000 lux or more. Most of us need to expose ourselves to this level of light for at least 30 minutes in the morning.