In humans, stage 4 sleep seems to be particularly important for repairing damage: Shapiro (1981) investigated the sleep of Ultra-marathon runners before and after they took part in a competition (Ultra-marathons are 57 miles). For the two days following the competition they slept 1½ hours longer. Stage 4 sleep was 45% of total sleep time rather than the usual 25%.

Further evidence for the role of stage 4 sleep comes from observations that a growth hormone that is important for RNA and protein synthesis, and hence tissue growth is released during stage 4 sleep. Also, when stage 4 sleep is prevented (by waking up participants whenever they enter this stage), they develop symptoms similar to those of fibrositis (severe inflammation of the back muscles). Moreover, fibrositis sufferers experience a chronic lack of stage 4 sleep.

Other evidence that demonstrates the importance of stage sleep comes from sleep deprivation studies such as the case of Randy Gardner (see Psychiatric Times article). While Gardner did sleep for longer after his ordeal (15 hours when he first fell asleep and longer than usual for a few nights), he only recovered about 25 hours of his sleep debt before reverting to his normal sleep wake cycle; nevertheless this was mostly stage 4 sleep (recovered 70%) and REM (recovered 50%).

Evidence against restoration theory

In an experiment where healthy participants spent six weeks in bed, Ryback and Lewis (1971) found no change in their patterns of sleep. Restoration theory, on the other hand, would predict that these participants would have less sleep, since less damage would occur if they are inactive.