Melatonin mystery退黑素的奥秘—真的能治疗失眠吗

To explore melatonin's relationship with sleep, it helps to understand how sleep is regulated. First, there is what is known as sleep pressure. The longer you are awake, the more a chemical called adenosine （腺苷）is released by cells in the brain. Adenosine inhibits arousal and increases the drive to sleep. Caffeine blocks adenosine receptors, which explains why it can help you temporarily overcome tiredness. Sleep pressure is balanced by the force of a 24-hour circadian rhythm, known as "circadian alertness", which is synchronised to the external environment and can be modulated by things like sunlight. Melatonin plays a key role in this synchronisation. Its synthesis is regulated by the brain's so-called master clock-the suprachiasmatic nucleus - to produce a circadian pattern of melatonin release, with levels rising at dusk, peaking between 2am and 4am and declining before dawn (see "The daily drive to sleep", below). The suprachiasmatic nucleus has receptors that detect melatonin, providing another cue for synchronisation. Sleep is an intricately choreographed process, easily disrupted by jet lag or working night shifts. Both of these are related to a mismatch between our innate circadian rhythms and the external environment. With jet lag, we cross time zones too rapidly for our circadian clock to adjust. The suprachiasmatic nucleus can adapt to a disrupted social schedule and the different timings of light and dark at our destination, but it takes a while - about one day for each nour or time zone change. With night shifts, sleep pressure may increase after an exhausting stint at work, but just as your head hits the pillow, your body clock may insist it is time to wake up. This isn't helped by environmental cues, such as quiet streets, closed shops and dark skies during your shift, telling you everyone else is asleep. No wonder melatonin was seen as a solution to sleep problems. Our urge to sleep closely correlates with the hormone's rise and fall, so people assumed it was the cause of sleep, says Russell Foster at the University of Oxford.