The hormone melatonin, produced by the pineal gland, is a sleep facilitator with circadian action. In the normal case, the inner clock in the suprachiasmatic nuclei (SCN) sends an “on” signal to the pineal gland sometime in the evening a couple of hours before we feel ready for sleep, and an “off” signal shortly before we wake up, after which the hormone washes out of the bloodstream and is absent until the following night.
Melatonin also talks back to the SCN, and influences its timing. This has inspired the use of melatonin tablets to “trick” the inner clock, when it is running late, to shift earlier. In turn, sleep onset shifts earlier, which can help people who are unable to fall asleep in time. But it is not a sleeping pill: it acts most powerfully taken hours before bedtime in a tiny dose that does not force sleep onset, but rather nudges the circadian clock, and its sleep onset signal, earlier.
It is helpful to think of melatonin administration just like light therapy, but taken at opposite times of day. In fact, both methods used together form a powerful circadian control signal that can move sleep earlier or later, depending on the sleeper’s goal.
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