A chronobiotic was first defined loosely as a drug that specifically affects some aspect(s) of biological time structure (60) and then more precisely as: a chemical substance capable of therapeutically re-entraining short-term dissociated or long-term desynchronized circadian rhythms, or prophylactically preventing their disruption following environmental insult (59) (see reviews elsewhere (20,55)).
MLT may be an internal zeitgeber (7,8) and as a rule of thumb, its effects on cir-cadian timing are the opposite to that of light i.e. when light phase advances a rhythm, MLT administration phase delays and vice-versa. Since the ability of MLT to entrain rat circadian rhythms depends upon the integrity of the SCN (17) and since both the rodent and human SCN contain high affinity MLT receptors (50,69), it would be reasonable to expect that exogenous MLT administration should be therapeutic for treatment of circadian insomnias.
With respect to MLT and treatment of circadian sleep disorders, the chronobiotic properties of the substance should be distinguished from its soporific/hypnotic effects
(56,73). In many studies claiming hypnotic effects, these have been confounded with chronobiotic effects. The extents to which the hypnotic properties are reliant on the hypothermic effects are still debatable (31,38).
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