"You get the most benefit from exercise when you move from no exercise to just a little exercise."
Whatever your exercise habits, you're more likely to have trouble sleeping if you sit too long during the day, the survey indicated. Respondents who sat for less than eight hours a day were twice as likely to say they had "very good" sleep quality than did those who sat for eight hours or more.
"If you spend the rest of the day sitting down, a lot of the health effects of exercise are negated," Dr Kline said.
No research before had connected sedentary behaviour with poor sleep, he said.
No difference in the quality of their sleep was reported by those who exercised at night and those who exercised earlier in the day.
"This was the finding that was somewhat surprising to me," said Dr Daniel Shade, a sleep specialist with Allegheny General Hospital in Pittsburgh.
"We would say you shouldn't exercise closer to bedtime than four to six hours. The thinking was you would be hyperactivated, hyperaroused."
As a result of this finding, he'll probably alter the advice he gives patients, Dr Shade said.
"You might tell insomniacs they should exercise earlier in the day," he said.
"But if you are normal, apparently it doesn't matter when you work out."
What's important is that the more you exercise, the better you'll sleep, "so we can tell our patients to keep moving", Dr Shade said.
In the poll, conducted by the National Sleep Foundation, 18 per cent of respondents described themselves as "vigorous" exercisers, 25 per cent as "moderate" exercisers and 48 per cent as "light" exercisers. Nine per cent said they didn't exercise at all.
Eighty-three per cent of vigorous exercisers, 77 per cent of moderate exercisers and 76 per cent of light exercisers - but only 56 per cent of non-exercisers - described the quality of their sleep as "fairly good'' or better.
The quality of their sleep improves on days they exercise, said 62 per cent of vigorous exercisers, 54 per cent of moderate exercisers and 49 per cent of light exercisers.
Eight per cent of vigorous exercisers, 14 per cent of moderate exercisers, 16 per cent of light exercisers and 24 per cent of non-exercisers said they had difficulty falling asleep. It took vigorous exercisers just 16.6 minutes, on average, to fall asleep, compared to 20.5 minutes for moderate exercisers, 22.6 minutes for light exercisers and 26.3 minutes for the sedentary.
Twice as many non-exercisers (34 per cent) as vigorous exercisers (17 per cent) take medicine to help them sleep.
Only 40 per cent of vigorous exercisers, as opposed to 46 per cent of moderate exercisers, 55 per cent of light exercisers and 72 per cent of the sedentary, reported feeling tired during the day. More than twice as many of the sedentary (14 per cent) reported having difficulty staying awake at least once a week while driving, eating or engaging in social activity than did those who exercised (four to six per cent).
Just 22 per cent of those who sat for less than six hours a day described their sleep quality as "fairly bad" or worse, compared to 25 per cent who sat for less than eight hours and 30 per cent for those who sat for more than 10.
- SNS
