Studies on exercise and sleep come up with the same conclusion time after time: If you want to hit the hay earlier and sleep better, get a good cardio workout.

But if you're already sleep-deprived, don't expect a 30-minute run or stint on the elliptical to knock you out quicker tonight.

The sleep-boosting effects of exercise can take a few months to kick in for people who suffer insomnia, scientists report Thursday in the Journal of Clinical Sleep Medicine.

"It's a long-term relationship," explains sleep psychologist Kelly Glazer Baron, who led the study. The benefits are worth the wait.

After four months, adding 30 minutes of exercise, three times a week, bumped up insomniacs' sleep by about 45 minutes each night, Baron says. "That's huge. It's as large or larger an effect than what you see with [sleep] drugs."

The current study is small. Baron and her colleagues at Northwestern University Feinberg School of Medicine tracked sleep and exercise patterns in just 23 women over the age of 55 who had been wrestling with chronic insomnia.

"But the analysis is powerful," she says, "because we had data for each woman every day for 16 weeks."

Baron generally treats people who have chronically lousy sleep with cognitive behaviorial therapy. But she also recommends regular exercise for all her patients — 30 minutes of cardiovascular exercise, three to four times a week.

"My clients were coming in and saying, 'I did what you said. I exercised so hard, but I didn't sleep very well,' " Baron says.

When she looked back at her data, she found that her clients were right: The amount of exercise during one day didn't correlate with how much the person slept that night. But the inverse was true. "How much they slept at night was linked to how much they exercised the next day," she says. "Sleep was driving the exercise."

So what does that mean for the rest of us exhausted workaholics?

"Not sleeping well makes you not want to exercise," she says. "But those are the days that you need to do it more. It will help you in the long run."

Several studies in the lab have found that working out right before bed may make it harder to fall asleep, Baron says. A recent survey by the National Sleep Foundation challenged that long-standing belief. But she still recommends that people who have chronic insomnia exercise more toward the middle of the day.

To get over the "too-tired-to-exercise" trap, Baron suggests planning your workouts in advance or leaving notes around the house reminding you it's worth the effort.

"I'm an exerciser," she says. "But I would just lie in the bed a little longer in the morning. So I have to pre-plan my workouts. You've got to just put on the shoes and get out the door."

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