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Delta sleep-inducing peptide (DSIP) is a naturally occurring substance, which was originally isolated from rabbit brain in 1977. As its name suggests DSIP promotes sleep and this has been demonstrated in rabbits, mice, rats, cats and human beings.
In fact DSIP promotes a particular type of sleep which is characterized by an increase in the delta rhythm of the EEG. DSIP is normally present in minute amounts in the blood. Brain and plasma DSIP concentrations exhibit a marked diurnal variation and there has been shown to be a correlation between DSIP plasma concentrations and circadian rhythm in human beings.
Concentrations are low in the mornings and higher in the afternoons. An elevation of endogenous DSIP concentration has been shown to be associated with suppression of both slow-wave sleep and rapid-eye-movement sleep and interestingly also with body temperature. Plasma concentrations of DSIP are influenced by the initiation of sleep.
Patients with Cushing’s syndrome suffer from a lack of slow-wave sleep but the diurnal variation in slow-wave and rapid-eye-movement sleep in those patients appears to be similar to that in normal patients.
When compared with most other peptides, DSIP is unusual in that it can freely cross the blood–brain barrier and is readily absorbed from the gut without being denatured by enzymes. DSIP is present in relatively high concentrations in human milk. Any mother who has breast-fed her babies will attest to the ability of a feed to induce sleep.
Since the discovery of DSIP a number of studies have been undertaken to test the hypothesis that DSIP may be the principal endogenous sleep factor. It is reported to increase the ‘pressure to sleep’ in human subjects who have been injected with small doses and this, together with its ability to induce delta-wave sleep, led to its consideration as a treatment for insomnia.
DSIP has been described as a sleep-promoting substance rather than a sedative. There is a modulating effect on sleep and wake functions with a greater activity in circumstances where sleep is disturbed. There are minimal effects in healthy subjects who are not suffering from sleep disturbance.
DSIP is not a night sedation drug which needs to be given just before retiring. A dose of DSIP given during the course of the day will promote improved sleep on the next night and for several nights thereafter.
DSIP may, paradoxically, be of use in the treatment of narcolepsy and it is possible that it exerts its effect by restoring circadian rhythms. When single and multiple injections of DSIP were given in a controlled double-blind study, disturbed sleep was normalized and better performance and increased alertness was seen during awake cycles together with improved stress tolerance and coping behaviour.