TONIGHT, as we all know, we will reset our mechanical and electrical clocks to coincide with GMT. But Although we are normally not aware of it, each of us has an internal "biological clock" which requires resetting almost every day. It governs, inter alia, our body temperature, sleeping and waking patterns, hormone levels, excretion rates and mental ability - all of which peak at different times throughout the 24 hour period.
Body temperature, for example, is at its lowest in the early hours of the morning, when we are usually asleep, and begins to rise again as the time when we normally get up approaches. This, and other "circadian rhythms", as they are called, are broadly attuned to the 24 hour day, but if we are deprived of the external stimuli that regulate the system, the rhythms quickly drift out of phase with the real world.
In an evolutionary context, the possession of a biological clock has allowed organisms to anticipate regularly recurring events rather than simply reacting to them, and has therefore given those possessing finely tuned examples a competitive advantage. Moreover, all circadian clocks have evolved mechanisms whereby they are frequently reset or entrained to be in phase with changes in the local environment. In the case of most creatures the light dark cycle of the 24 hour day is the most powerful stimulus, but other cues like feeding regimes and social activity also play a part.
The physiological responses themselves continue in the absence of any external stimuli but often with a slightly altered cycle. In the case of humans, for example, it has been found that if volunteers are deprived of any perception of the light dark cycle, or any other means of reckoning time, they continue to have a steady routine of biological rhythms, but one based on a cycle of about 25 hours, or even longer. As with many other species, exposure to daylight is the major factor for humans in the regular resetting of the biological clock; without this stimulus, our clocks run slow.
Some solitary blind people, for example, or people living indoors without social contact, get up and go to bed later and later each day, losing their synchronisation with the rest of society. Teachers and parents, moreover, are well aware that strong clock resetting signals in the morning are necessary for many young people who, left to their own devices, tend to become desynchronised over long weekends or during the school holidays.