Increased numbers of workers pull the night shift. Teenagers text past midnight and stumble to class at dawn. Travelers pack cheaper overnight flights.

Nodding off behind the wheel is not the only threat from a lack of sleep. Evidence is growing that people who regularly sleep too little and at the wrong time suffer long-lasting consequences that a nap will not cure: increased risk of diabetes, heart disease and other health problems.

``We have a societal conspiracy for sleep deprivation,'' says Russell Sanna of Harvard Medical School's sleep medicine division, who attended a TEDMED conference last week where scientists called sleep loss one of health care's big challenges.

Just how unhealthy is it? Consider how sleep may play a role in the nation's diabetes epidemic.

Studies have long shown that people who sleep fewer than five hours a night have an increased risk of developing Type 2 diabetes, the kind that tends to strike later in life.

Rotating shift work, three or more night shifts a month interspersed with day or evening hours, raises the risk, too, says a recent report from researchers who analyzed years of medical records from the huge Nurses' Health Study.

Diet and physical activity are big factors in Type 2 diabetes. Certainly it is harder to work out or choose an apple over a doughnut when you're tired, especially at 3 a.m. when your body's internal clock knows you should be sleeping.

A study published last week, however, shows sleep plays a more complex role than that. As sleep drops and normal biological rhythms are disrupted, a person's body physically changes in ways that can help set the stage for diabetes, reports neuroscientist Orfeu Buxton of Boston's Brigham and Women's Hospital.

Buxton's team had 21 healthy volunteers spend almost six weeks living in a laboratory where their diet, physical activity, sleep and even the light was strictly controlled.

The volunteers started out well-rested. But for three of those weeks, they were allowed only about 5{ hours of sleep every 24 hours _ at varying times of the day or night, to mimic a bad shift rotation or prolonged jet lag. That knocked out of whack the body's ``circadian rhythm,'' a master biological clock that regulates such patterns as when a person becomes sleepy or how body temperature rises and falls.

What happened was startling: Blood sugar levels increased after meals, sometimes to pre-diabetic levels, because the pancreas stopped secreting enough insulin, Buxton reported in the journal Science Translational Medicine.

At the same time, the volunteers' metabolic rate slowed by 8 percent. The researchers had them on a diet so they did not gain weight, but Buxton says typically, a metabolism drop of that size could mean gaining 10 to 12 pounds over a year.

The results make sense, says Dr. Michael Thorpy, sleep center director at New York's Montefiore Medical Center and a neurology professor at Albert Einstein College of Medicine.

``If we're going to spend a third of our day sleeping, there's got to be a good reason for it,'' says Thorpy, who notes that diabetes is far from the only worry.

Up to 70 million Americans are estimated to suffer from chronic problems with sleep, from insomnia to sleep apnea. Impaired sleep has been linked to high blood pressure, heart disease, obesity, depression, memory impairment and a weakened immune system. Still another concern: The World Health Organization has classified night shift work as a probable carcinogen, because too much light at night may hamper a hormone involved both with sleep and suppressing tumor cells.

Don't people adjust to the night shift if they are on it long enough? Buxton says rotating shifts probably are most worrisome. In his study, the volunteers' bodies went back to normal after nine nights of sufficient sleep at the right time. No one knows how long it takes before sleep deprivation and an off-kilter biological clock may cause permanent damage.

Montefiore's Thorpy says natural night owls seem to adapt better to night shifts, but that people never fully adapt if they swing back to daytime schedules on their days off. Also, about 30 percent of regular night workers have trouble sleeping during their off hours or are particularly fatigued, he says, something termed ``shift work disorder.''

The consumer message:

_The National Institutes of Health says adults need between seven and nine hours of sleep daily for good health.

_People you work nights, should  go straight to bed when they get home, Buxton advises. Avoid too much light along the way. Thorpy says wearing yellow- or orange-tinted sunglasses on the drive home can block short-wavelength ``blue light'' that triggers wakefulness.

_Let natural light help keep the biological sleep clock on schedule, advises Harvard's sleep-education Web site. For most people, sunlight in the morning is crucial. For the night shift, more bright light in the evening shifts people's internal clock, Buxton explains.

_For anyone, a sleep-inducing bedroom is one that's dark, quiet and cool. Avoid caffeine, alcohol and stressful situations near bedtime. Electronics right before bed aren't advised, either. Going to bed and waking up at the same time every day also helps.