All plants and animals have a built

All plants and animals have a built-in biological clock - known scientifically as circadian. rhythms - which is approximately synchronised to the 24-hour rotational cycle of the Earth. Put simply, it's the body's wake / sleep cycle. The sleep part is driven by a hormone called melatonin, which is produced in the pineal gland in the brain. For diurnal animals, like humans, melatonin production starts in the evening, reaching a peak in the middle of the night, and stops at around 5 or 6 a.m. - it's what makes us tired at night and alert in the morning (or most of us anyway). It's also the reason for jet lag when crossing time zones too quickly, This body clock doesn't completely depend on light as other things influence it too, such as mealtimes and temperature, but without light an individual's circadian rhythm can become seriously unbalanced, affecting mental and physical health. Daylight cycles are the most effective way of resynchronising the biological clock.