Every morning I am greeted by Facebook friends complaining of sleepless nights or awakenings. I know the feeling — as do many other people.

In a 2005 survey of 1,506 Americans by the National Sleep Foundation, 54 percent reported at least one symptom of insomnia — difficulty falling asleep, waking up a lot during the night, waking up too early or waking up feeling unrefreshed — at least a few nights a week over the previous year. Thirty-three percent said they had experienced symptoms almost every night.

If insomnia visited me that often, I’d be tempted to pick up something at the pharmacy — something easy, something safe, something that didn’t involve making a doctor’s appointment. Indeed, 10 percent to 20 percent of Americans take over-the-counter sleep aids each year, according to the American Academy of Sleep Medicine.

The way they’re marketed, over-the-counter sleep aids sound very appealing: The new product ZzzQuil (from the maker of NyQuil) promises “a beautiful night’s sleep;” an ad says you’ll “fall asleep faster and stay asleep longer” after using Unisom. Companies marketing the herb valerian root and the hormone melatonin as over-the-counter sleep aids make similar claims.

But what’s the evidence that supports these claims? “It’s quite lean,” says Andrew Krystal, who directs the sleep research program at Duke University.

Over-the-counter sleep aids work differently from prescription drugs for insomnia. Most are simply antihistamines in sheep’s clothing. (Yes, that’s a joke.) The majority of them contain diphenhydramine as the active ingredient, the same compound in Benadryl.

The clinical studies testing diphenhydramine for insomnia are minimal, Krystal says: There have been only two, which together involved 204 people. “That’s it. That’s all we have,” he says. (Other studies have looked at daytime sleepiness with the drug.)

The studies looked at people with primary insomnia — meaning that their sleep problems were not the result of other medical issues such as depression, anxiety or pain.

The first study included a comparison of the effects of diphenhydramine (50 mg, a typical OTC dose) with those of a placebo in 20 elderly people with insomnia. The participants reported slightly fewer nighttime awakenings with diphenhydramine than with the placebo, but no difference in how long they took to fall asleep, how well they slept or how long they slept.

The second study tested 25-mg doses of diphenhydramine against a placebo and an herbal preparation of valerian and hops in 184 adults with mild insomnia. Compared to a placebo, diphenhydramine improved sleep efficiency (the percentage of time in bed spent sleeping) based on participants’ feedback but not on automated readings of brain, eye and muscle activity. Neither did it affect sleep onset or total sleep time.

What these studies did find with diphenhydramine was side effects, including dry mouth, dizziness and headache. Other side effects that can occur with diphenhydramine are constipation and urinary retention, Krystal says.

The valerian-hops combination helped, to some degree: People who took that mixture reported that it took slightly less time to fall asleep and that their insomnia was less severe.

The problem with valerian root is that there’s so much variety in the preparations, says Vivek Jain, medical director of the Center for Sleep Disorders at George Washington University Hospital in Washington. You can’t be certain how much active ingredient you’re getting, he says, and because these products are regulated as supplements rather than drugs, their composition can vary from one maker to the next.

Krystal concurs. “You never know what you’re getting. It’s an extract from a root. …No two batches will be the same.”

The bottom line is that the evidence has not shown more than modest effects of valerian, Krystal says.

What about melatonin? Jain says there is some evidence that it helps people fall asleep more quickly. A 2011 study of prolonged-release melatonin found that insomnia patients age 55 to 80 fell asleep 15 minutes sooner than with placebo, on average, but younger patients did not benefit. Krystal says that melatonin is helpful for shifting one’s day-night cycle, for people changing time zones or work shifts and for helping night owls go to sleep at what is for them an unnaturally early time.

There’s a larger reason why sleep aids don’t work, Jain says, and that’s because insomnia is not just a nighttime phenomenon. “It’s a 24-hour problem,” he says. Our wakeful state is akin to second or third gear in a car with a five-gear transmission. At night we drop into first gear or neutral.

“In insomnia, these gear shifts get unstable,” Jain says. Insomniacs spend their days in higher gears than most people, and they have trouble downshifting at night. “Patients don’t recognize the daytime problem. They just want to sleep more and more,” he says.

When considering sleep aids, he cites the lack of benefit and the risk of side effects and says, “There’s no good reason to take these. Ever.”

If you are going to use them, he says, “use them intermittently — not every night.”

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