The scientists studied 149 healthy men ages 16 to 83. They found that the proportion of deep sleep drops from nearly 20% of a normal night's sleep for men under 25 to less than 5% for those over 35. By age 45, few men spend much time in deep sleep.

Thomas Roth, sleep medicine chief at Henry Ford Hospital in Detroit, calls [Eve Van Cauter]'s report "very important" because it shows that the decline in deep sleep begins at a relatively young age and that it is strongly correlated with a drop in growth hormone levels.

Van Cauter says she and her collaborators are just beginning to collect data on deep sleep in women, who are trickier to study because of the effects of estrogen on their sleep patterns. And, Van Cauter says, while two-thirds of men's growth hormone production occurs during deep sleep, only about a third of women's does.