Periodicity Health Dictionary

This is characterised by the presence of rapid eye movements and a reduction in muscle tone. Cerebral cortical activity is prominent and its blood ?ow increased. This activity is, however, di?erent from wakefulness and may cause irregular movements of the body as well as of the eyes. Most dreams occur in REM sleep: these may represent a process of reorganising mental associations after the period of wakefulness. The analysis of the content of dreams has been subject to a variety of interpretations, but no consensus view has evolved.

Physiological changes, such as a fall in temperature and blood pressure, take place just before sleep and continue during the early stages of NREM sleep. There is an intrinsic rhythm of sleep which in most subjects has a periodicity of around 25 hours. This can be modi?ed by external factors to bring it into line with the 24-hour day. Two peaks of a tendency to sleep have been identi?ed, and these usually occur between around 14.00–18.00 hours, and 02.00–06.00 hours. There are, however, di?erences according to age, in that, for instance, infants sleep for most of the 24 hours; during adolescence there is also an increase in the duration of sleep. Sleep requirements fall later in life, but there are wide genetic di?erences in the amount of sleep that people require and also the time at which they fall asleep most readily.

The internal clock can be disturbed by a variety of external factors which include irregular sleeping habits due, for instance, to shift work or jet lag. Sleep is also more likely to occur after physical exertion, reading and social activity. The duration and intensity of exposure to light can also modify sleep profoundly. Light promotes wakefulness and is the main factor that adjusts the 25-hour internal rhythm to the 24hour daily cycle. Neural connections from the retina of the EYE act on an area in the brain called the supra-chiasmatic nucleus which stimulates the pineal gland which produces MELATONIN. This is thought to trigger the range of neurological and metabolic processes that characterise sleep.... rapid-eye-movement (rem) sleep

Recurrence of malarial parasitaemia with fresh infection of RBC’s by merozoites derived from hypnozoites in the liver. The reappearance of a disease after a period when the symptoms lessened or ceased. A renewed manifestation of clinical symptoms and/or parasitaemia associated with malaria infection, separated from the previous manifestation by an interval greater than the one reflecting the normal periodicity of paroxysms.... relapse