The Body Clock And The Sleep / Wake Cycle (circadian Rhythm)
The body clock comprises a tiny cluster of nerve cells sitting in the hypothalamus, which forms the base of the brain. This group of a few thousand nerve cells switch on and off groups of genes in a feedback loop lasting 24 hours. The expression of these genes alters the activity of the nerve cells which therefore effectively 'keep time'. The time registered by the body clock is then used to control the release of other hormones which are concerned with preparing us for activity or rest respectively, and in controlling the parts of the brain involved with alertness and arousal.
Luckily we don't suffer permanent jetlag when we go abroad because the clock can be reset by exposing our eyes to light. A small number of nerve fibres from a specialised group of light-responsive cells in the retina feed in to the body clock and can adjust the rhythm, according to whether it is light or dark, just as one would alter a wristwatch. This system allows the clock to adapt to changes in day length and the demands of modern-day living, but the system is not perfect and does not kick in immediately, which is why we feel jet-lagged after a long haul flight, or after working a night-shift.