Circadian rhythm periodicity
Circadian rhythms have a periodicity of around a day, which is longer than ultradian rhythms of,  for instance, REM sleep and shorter than infradian rhythms such as the menstrual cycle. The intrinsic circadian rhythm ranges between 23.5 and 24.5h with a mean of around 24.2h in humans. Approximately 25% of subjects have a rhythm which is less than 24h and 75% a rhythm longer than this.

The circadian rhythms are generated by an internal pacemaker, oscillator or biological clock whose activity is modified by external factors (time givers, cues or zeitgebers). These reset or entrain the internal clock and gear it to the external environment. In certain situations the clock can become dissociated from the time givers, in which case it becomes ‘free running’ and desynchronized from the environment.

Suprachiasmatic nuclei (SCN)
The suprachiasmatic nuclei (SCN) in the supra-optic region of the anterior hypothalamus are the centres responsible for the most important circadian rhythms.

These two tiny bilateral nuclei about 3cm behind the eyes each have a volume of only 0.1ml and each contains around 10000 neurones.

Each SCN has ‘a core’ of neurones which secrete either vaso-intestinal peptide or gastrin-releasing peptide. They respond particularly to light stimuli via the retino-hypothalamic tract and have melatonin receptors. The ‘shell’ of arginine vasopressin- and calretinin releasing neurones responds to non-photic stimuli.

This region probably controls the circadian elements of motor activity and feeding behaviour. Gamma-aminobutyric acid (GABA)-secreting neurones are present in both the core and the shell.

Each of the cells in the SCN is capable of spontaneous depolarization. The coordination of these is the source of circadian rhythmicity, but within the SCN there are subpopulations of cells with different cycle times _[16]. These are usually entrained by diffuse chemical, rather than synaptic, mechanisms to a single rhythm but under certain circumstances they can become dissociated from each other.

The SCN is active during the day, both in diurnal and nocturnal animals. In humans it promotes wakefulness and influences the structures which control the onset and maintenance of sleep, but is not solely responsible for this.

By depriving people of light and other external time cues, scientists have learned that most people’s biological clocks work on a 25-hour cycle rather than a 24-hour one. But because sunlight or other bright lights can reset the SCN, our biological cycles normally follow the 24-hour cycle of the sun, rather than our innate cycle. Circadian rhythms can be affected to some degree by almost any kind of external time cue, such as the beeping of your alarm clock, the clatter of a garbage truck, or the timing of your meals. Scientists call external time cues zeitgebers (German for “time givers”).

Many people with total blindness experience life-long sleeping problems because their retinas are unable to detect light. These people have a kind of permanent jet lag and periodic insomnia because their circadian rhythms follow their innate cycle rather than a 24-hour one. Daily supplements of melatonin may improve night-time sleep for such patients. However, since the high doses of melatonin found in most supplements can build up in the body, long-term use of this substance may create new problems. Because the potential side effects of melatonin supplements are still largely unknown, most experts discourage melatonin use by the general public.
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