From “The Sleeping Cure”, by Richard A. Friedman, New York Times, March 12, 2017:
When you quickly cross several time zones, your circadian rhythm remains stuck in the city you left behind. Arriving in Rome with your New York City brain is what produces the unpleasant symptoms of jet lag: fatigue, malaise, poor concentration and mood changes.
When you leave New York at 6 p.m., the Italians are probably in bed asleep. But you won’t feel ready for sleep until around 11 [p.m. New York time, or around 5 a.m. in Rome]. To make the right adjustment, you need to shift your internal clock earlier by six hours.
Unfortunately, exposure to light in the middle of the night will do the opposite. Instead of shifting you earlier to Italian time, it makes you feel it’s even later — that the night is over and it’s already morning.
If you’re ever in that situation, close the shades and put on dark sunglasses. Keep the glasses on until lunchtime in Rome — or 7 a.m. back home. Then go out into the sun, have an espresso and enjoy the splendor of the ancient city. This will shift your clock closer to Roman time.
The clock in your brain doesn’t just take cues from light, but from the hormone melatonin as well. Every night, about two to three hours before you conk out, your brain starts to secrete melatonin in response to darkness. Taking a melatonin supplement in the evening will advance your internal clock and make it possible to fall asleep earlier; taking it in the morning will do the opposite. (You might assume this would make you even more tired during the day but it won’t; you could think of it as tricking your brain into believing you slept longer.)
So now you know the fix for jet lag: Travel east and you’ll need morning light and evening melatonin; go west and you’ll need evening light and morning melatonin.
Researchers have developed a limited form of sleep deprivation that is euphemistically called wake therapy. It has been shown to have sustained antidepressant benefit in patients with bipolar disorder and major depression. The idea is to get up for the day halfway through the usual sleep period, which shifts the circadian clock to an earlier time. It’s thought that this works by realigning the sleep cycle with other circadian rhythms, like changes in levels of body temperature and the stress hormone cortisol, that are also out of sync with each other in depression.
Studies show that it is possible to make wake therapy even more powerful by incorporating two additional interventions: early morning light therapy and what’s called sleep phase advance, in which the patient goes to bed about five to six hours earlier than usual and sleeps for about seven hours. This combination of treatments is called triple chronotherapy, and the typical course involves one night of complete sleep deprivation followed by three nights of phase-advanced sleep and early morning light.