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Don’t people adjust to the night shift if they’re 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.

_If you work nights, go straight to bed when you 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 your biological sleep clock on schedule, advises Harvard’s sleep-education Web site. For most people, sunlight in the morning is key. 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.


EDITOR’S NOTE _ Lauran Neergaard covers health and medical issues for The Associated Press in Washington.



Harvard sleep education:

NIH Guide to Healthy Sleep: