IMPORTANT NOTE: This is not a medical site. You should always discuss medical matters with your doctor.
The Circadian Rhythm
The term circadian rhythm is simply a technical way of describing our body's natural daily rhythm - our internal body clock. The name comes from the Latin "circa" (around) and "dies" (day).
It's actually over simplisic to talk about "the" circadian rhythm. Many activities of our body are influenced by daily rhythms of some sort. Scientists have identified over 100 different rhythms including blood pressure, body temperature and heart rate. These different circadian rhythms interact to keep our bodyies on track - or otherwise. It has been reported that heart attacks and strokes are more common in the morning than at other times of day.
The exact mechanisms behind all these circadian rhytms are unclear, however they appear to involve hormone production in many cases. How is this hormone production timed to a daily cycle? The most likely candidate as a control mechanism is daylight, and studies have now confirmed this. However the clock is not wholly dependant on external factors as it takes time to "reset" if the external light cues are changed. This delay can be one cause of jet lag.
The Sleep-Wake Cycle
Of the various different circdian rhythms, the one of most interest to this site is the daily sleep-wake cycle. This cycle is approximately 24 hours long - however in many cases it appears to be slightly longer than this, closer to 25 hours. The evolutionary reason for this discrepancy is not known.
In humans and other mammals the circadian rhythm is mainly controlled by the hypothalamus. In particular there is a group of cells called the supriachiasmatic nucleus (SCN) which, if destroyed, eliminates any detectable circadian rhythm.
The SCN appears to be activated by the arrival of light on the retina of the eyes. This then causes it to activate the pineal gland which produces melatonin. This hormone then adjusts the functioning of our body.
As we age this process becomes less reliable and our circadian rhythm breaks down. Older people often have unusual or disrupted sleep cycles that can be difficult to correct. There are also other disorders that can result in disturbed sleep and other problems - as always you should discuss such things with a doctor.
If the body's clock is regulated b light, it follows that a change in the natural daily cycle of light-dark patterns can disrupt our hormonal balance. In some people the lack of daylight during the winter months results in a condition known as Seasonal Affective Disorder (SAD). Many sufferers of SAD use light therapy, including lightboxes and dawn simulation alarm clocks.
References:
Circadian Rhythms