What's more important: an extra hour of sleep or getting up early to exercise? How do I balance an extra hour of sleep against 40 minutes of exercise? When should I give in to the desire for more sleep and when should I push myself to exercise? Both are good for you!

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Ah, a predicament familiar to many of us, especially on cold, dark or rainy mornings when a warm bed can seem insidiously alluring compared with an early jog. Unfortunately science can’t help us definitively to balance one activity against the other.

“Exercise, sleep and nutrition form the triangle of health, and all are related,” said Dr. Phyllis Zee, a professor of neurology and director of the Sleep Disorders Center at Northwestern University in Chicago.

Research from her laboratory shows that a good night’s sleep, consisting of at least seven hours of slumber, results in better and more prolonged exercise sessions later that day, she said, while fewer hours of sleep frequently lead to reduced motivation to exercise. Similarly, “exercise can improve the quality of sleep,” she said, prompting “deep sleep that is more restorative and effective for memory, performance and physical health.”

In other words, she said, sweating and slumber share a “bidirectional relationship.”

Because of this intertwining impact, robbing yourself of exercise or sleep is counterproductive to good health, said Kelly Glazer Baron, the director of the Behavioral Sleep Medicine Program at Northwestern. Better to rejigger your schedule.

“Look at your life and figure out what you can swap for exercise and still keep your sleep,” she advised. A full-time academic and mother of 2-year-old twins, Dr. Baron bikes to work or runs at lunchtime and also believes in interval training, which involves intense but brief bouts of exercise.

As for how to bank more sleep without skipping gym time, consider simply “going to bed 20 to 30 minutes earlier than usual on work days,” Dr. Zee said, and “waking up 15 minutes later in the morning,” earning yourself an additional 600 to 800 minutes of sleep per month. “Everyone can do that,” Dr. Zee said.