The use of melatonin in jet lag

Circadian rhythm or body time is the major biological endogenous rhythm (sleep, body temperature, cortisol production) and is generated and coordinated by a circadian master oscillator located in the anterior hypothalamus area, the suprachiasmatic nuclei (SCN) (1). Human circadian rhythms have a free-running period of approximately 24.5 hours (range 22.5–27.5 hours (2), however, normally the rhythm is entrained to the exact phase length of the external day-night cycle by environmental time cues (or ‘zeitgebers’). Melatonin (N-acetyl-methoxytryptamine), a methoxyindole, is the primary hormone secreted by the pineal gland. The pineal gland occupies the depression between the super colliculi of the mesencephalon, and has a mass of 50 – 150 mg (3). Melatonin is synthesised from the amino acid tryptophan, via the intermediate serotonin. Activity of the enzymes involved in this synthesis is high during the dark phase of the day. As such, melatonin production occurs almost exclusively at night under normal conditions. Transmission of informa-tion occurs from the retina to the SCN and then to the reticular system, spinal cord and cervical ganglia. From there, post-ganglionic sympathetic fibres reach the pineal gland. The number of ?-receptors rises during the latter part of the light phase, noradrenergic stimulation begins in the dark phase and down regulation of ? receptors occurs through the course of the night (3). The secretion of melatonin itself starts as soon as darkness falls and usually peaks between about 02:00 and 04:00 hours. Since the duration of melatonin secretion depends on the duration of darkness, a greater amount of melatonin in secreted over 24 hours in Winter than in Summer.