Philip Gehrman is a psychologist at the University of Pennsylvania who studies the intersection of sleep and mental illness, a hot topic in treating everything from depression to bipolar disorder to PTSD.

"Yeah, I'm depressed," patients will often tell him. "If I could just sleep better, my mood would be fine."

He thinks some of them may be right.

Experts say doctors have long recognized that insomnia or changes in sleep "architecture" - the length and quality of different stages of sleep - are symptoms of virtually all mental illnesses. They're also finding that poor sleep heightens the risk of developing mood disorders, or of having trouble recovering from them.

Increasingly, researchers are testing whether improving sleep can treat or prevent mental illnesses.

David Kupfer, a prominent psychiatrist at the University of Pittsburgh, said the growing interest in sleep dovetailed with increasing fascination with neuroscience and circadian rhythms.

"Everybody has now become interested in where does sleep go and where doesn't it go," he said, "and it becomes a wonderful barometer of basic health."

The recognition that adequate sleep is crucial for physical and mental health has important repercussions in our frantic, 24/7, computer-screen-lighted society. Studies show most people need seven to nine hours of sleep a night, but the average amount we get has been declining. A third of the population sleeps less than six hours a night.

The world seems full of people who swear they barely need sleep. When was the last time you heard someone bragging about needing eight hours of sleep - and making it a priority to get it?

"The number one thing that suffers is our sleep," said Karl Doghramji, a psychiatrist and sleep expert at Thomas Jefferson University. Sleep, he said, "is critically important," and we undervalue it.

The message from sleep experts is that we're putting ourselves at risk when we treat sleep as optional or ignore problems sleeping. You may not be the best judge of how much sleep you need. Some of us are acutely aware of fatigue; others blunder on without realizing their thinking ability and mood are faltering.

"People who are more sleepy," Doghramji said, "tend to be less likely to identify their levels of sleepiness."

In recent years, word has been getting out about the physical consequences of disrupted sleep-wake cycles. In a recent paper on racial disparities in sleep - a possible factor in health disparities - Penn sleep researcher Michael Grandner said sleep restriction was associated with weight gain and obesity, hypertension, high cholesterol, inflammation, diabetes, stroke, heart attack, and death.

That's quite a list. But mental health experts say it's incomplete.

"What's interesting about sleep," said University of Pittsburgh psychologist Ellen Frank, "is that disordered sleep can be both a cause and a symptom of various mental problems, including bipolar disorder, unipolar depression, post-traumatic stress disorder, and a whole host of other mental disorders."

"The greatest risk," Gehrman said, "is for people habitually sleeping less than six hours a night. You name the negative outcome, and it's there."

Normalizing sleep patterns is a key part of a therapy for bipolar disorder that Frank developed: Interpersonal and Social Rhythm Therapy. People with bipolar disorder seem to have especially fragile natural clocks and do best with fairly rigid schedules, she believes. Many are natural night owls and function best when they go to sleep late and get up late. They can be thrown off by jet lag or emergencies that force them to miss sleep.

"You have a particularly vulnerable clock," Frank tells patients. "They need to be much less chaotic than the average person."

Scientists are still trying to figure out how sleep and mood are related. People with mood disorders are considerably more likely than others to have trouble sleeping. Insomniacs are also at higher risk for developing mental illness, as are people with sleep apnea and restless leg syndrome, which disrupt sleep. Insomnia can be a better predictor than depression of suicide.

Doctors once thought of insomnia - defined as taking more than 30 minutes to fall asleep or being awake 30 minutes during the night for at least three nights a week - as either an independent problem or a symptom of another disorder, such as depression, that would go away when the underlying disorder improved. But, Doghramji said, in about 40 percent of patients, it doesn't go away. Those are the patients most prone to relapse. Insomnia could also be a very early signal of a mood disorder, or it's possible the sleeplessness and mood problem share the same cause.

Sleep experts said insomnia could become a learned behavior. Maybe you initially couldn't fall asleep because you were anxious, but eventually you learned, Grandner said, that bed was a place for "tossing and turning and worrying and being uncomfortable." A sure sign that's the problem is when you can't fall asleep in bed but have no trouble on the couch.

Though medications can be used in crises, the experts said they preferred behavioral changes because those lead to better-quality sleep.

During sleep, Grandner said, our brains process and organize our memories. Memory and emotion are inextricably linked. When we don't get enough sleep, our brains can't do what he calls "proper maintenance."

At the Western Psychiatric Institute and Clinic in Pittsburgh, child psychiatrist Rasim Somer Diler uses special watches that monitor sleep in young inpatients with bipolar disorder. Sleep is a big part of his patient assessment. In bipolar disorder, a few days of short sleep - especially if the patient doesn't feel tired - may be a sign a manic episode is near.

Diler's young patients have to go to bed between 9:30 and 10 p.m. A poster on the unit lists tips for good "sleep hygiene." He also suggests doing three things in the same order - maybe brushing teeth, putting on pajamas, and washing your face - about an hour before bed each night. "It's like training your dog," he said.

One small but intriguing study from Ryerson University in Toronto found that successfully treating insomnia with talk therapy almost doubled the number of people who recovered from depression. A bigger study is in the works.

Frank has begun a study to see whether training young people at risk of bipolar disorder to make sleep a priority can prevent the disease. It will take years to get results, but Frank has been impressed that teenagers, who face a lot of peer pressure to stay up late and miss sleep, are listening.

"What we have seen is that they seem to get the message," she said, "and, contrary to what we expected, were relatively accepting."

Gehrman now hopes to study whether treating insomnia first can improve outcomes in PTSD.

Grandner has a particular interest in short sleepers, those people who say they don't need as much sleep as the rest of us. He will soon release data on their mental health but for now can only hint that many people who think they can get by on less sleep actually have problems.

Of course, many of us have noticed lack of sleep can make us cranky, even if we're not diagnosable. Irritability, Grandner said, is "mostly an inability to regulate emotion properly, and sleep is one of the most important parts of the system that regulates emotion properly."

He sees the relationship fallout frequently. "I see a lot of guys in clinic who can't keep relationships, who can't keep jobs, who don't like people," he said. "They think it's their personality."

A lot of that changes when they start sleeping properly. "Then," he said, "they have to start convincing their friends they're not a jerk."