Washington ? Millions of Americans lie awake at night counting sheep – or have a stiff drink or pop an allergy pill, hoping it will make them drowsy. But experts agree all that self-medicating is a bad idea, and the causes of chronic insomnia remain mysterious.

Almost a third of adults have trouble sleeping, and about 10 percent have symptoms of daytime impairment that signal true insomnia.

Sufferers readily cite the resulting problems: walking around in a fog, as memory and other cognitive functions slow. Dozing off at the wheel or at work. Depression. Lack of energy.

But for all the complaints, scientists know surprisingly little about what causes chronic insomnia, its health consequences and how best to treat it, a panel of specialists brought together by the National Institutes of Health concluded Wednesday.

Two things are clear, the panel found: Chronic insomnia is a major public health problem. And too many people are using unproven therapies, even while there are a few treatments that do work.

Among the panel’s findings:

¢ Newer prescription sleep pills called Sonata, Ambien and Lunesta work without many of the side-effect concerns of older agents known as benzodiazepines. One study of Lunesta showed effectiveness with six months of use, but more research on long-term use of all three is needed, as chronic insomnia can linger for years.

¢ The most commonly used treatments are alcohol and over-the-counter sedating antihistamines like Benadryl. Alcohol use actually disrupts quality sleep, and antihistamines can cause lingering daytime sedation and other cognitive problems.

¢ There is no evidence backing the effectiveness of the popular dietary supplements melatonin and valerian to fight insomnia.