An early section of Wide Awake (2006), Alan Berliner’s documentary about his life-long insomnia — he can’t fall asleep until 3 or 4 am — lists common folk remedies:
BERLINER Over the years I’ve tried to cry myself to sleep, to drink myself to sleep, aroma therapy, changing mattresses, changing pillows, lavender beads, massage therapy, white noise, meditation, counting sheep, melatonin, Valerian root, acupuncture, acupressure, chamomile tea, warm milk, hypnosis even, yoga, homeopathic medicines, marijuana, lots of sex, hot baths, herbal teas, biofeedback. SISTER Okay, nothing worked.
Conspicuously absent: sunlight. At the end of the movie, however:
DOCTOR We have to reset your [internal] clock. Since you’re such a night owl, I’d like to move your sleep cycle earlier by having you get light exposure in the morning. When you wake up, throw on some clothes and go outdoors for an hour. I really want light to get into your eyes ’cause that’s what going to move your rhythm so you can fall asleep earlier. ANOTHER DOCTOR Light is one of the most powerful cues for your internal clock to know what time it is. You see light and it tells you: be active during the day, sleep at night.
But the treatment they settle on is sleep deprivation: “I’d like you to spend just 6.5 hours in bed,” says a doctor. “Give you less time in bed than you want. . . . 2:30 to 9:00 am would be a reasonable way to go.” “You are going to be dysfunctional,” Berliner is warned. The film ends: “Now that I know what I have to do, the question is: Can I do it?”
This is a good summary of what people believe about how to cure insomnia. Sunshine is absent from the folk remedies you are likely hear. When doctors mention it, they emphasize early-morning sunlight.
Until recently, I too thought that sunlight exposure was important in the morning, but not during the rest of the day. Every morning I exercised on a treadmill with sunlight-spectrum light shining on me for an hour; I thought that was enough. Now I am adding to that sunlight later in the day — in the afternoon, for instance — and finding that it helps.