A recent study has found that no matter how much a person works out during the day, their exertion fails to translate into increased total sleep time at night.
The researchers observed that days with increased activity were followed by nights with lower total sleep time (TST), while nights with lower TST were followed by increased activities during the next day.
The findings indicated that total sleep time increased by an average of 42 minutes a night only after days with low activity.
The research team also saw increased activity in participants with higher body mass index (BMI).
Lead author Dr. Arn Eliasson, at the Integrative Cardiac Health Project at Walter Reed Army Medical Center in Washington DC, said that the results of the study were quite contrary to what they had expected.
Quality of sleep-measured by sleep efficiency or sleep time divided by time in bed-did not improve after days of increased exertion and sleep efficiency did not vary according to the amount of exertion during the day.
"It has long been recommended, even championed, that getting exercise is part of the recipe for improved sleep. Our data do not support that notion," said Eliasson.
He added: "The longest sleep and best sleep efficiency occurred after days with low non-exercise exertion. Similarly, we expected that better-rested subjects would be more inclined to get exercise or have busier days; however, bett er rested subjects got less exercise and had less calorie expenditure. After relatively more sleep (more than six hours), all measures of exertion decreased."
The study was conducted on 14 subjects who wore actigraphy armbands that measure body temperature, ambient temperature, position sense, and accelerometry for approximately 23 days.
The researchers also collected data regarding total sleep time, sleep efficiency, total energy expenditure, exercise energy expenditure, non-exercise activity steps and BMI.
When sorted by BMI, seven subjects had a normal BMI (under 25 kg/m2) and seven were overweight (over 25 kg/m2).
To their surprise, the researchers observed that the group with higher BMI had higher total energy expenditure, and took more steps per day.
Eliasson speculated that these findings may be explained by personality types: Individuals who are Type A (ambitious, active people during the day), may also be more hyper-vigilant at night and therefore sleep less; whereas people who are Type B (lower-key people who are less active) may have no difficulties falling or staying asleep.
One more explanation could be that job and life stresses lead to busier days, more exertion and more calories burned but may interfere with sleep.
The study was presented on Monday at SLEEP 2009, the 23rd Annual Meeting of the Associated Professional Sleep Societies.