The National Sleep Foundation recommends 7-9 hours of sleep daily, but for some of us with night shifts or early mornings, 7-9 is difficult to achieve. Instead we recharge our heads and our willpower with naps. Naps aren’t just for the sleep-deficient–growing experimental evidence indicates everyone can significantly benefits from getting midday shut-eye. But just how long should our siestas last?
Tietzel et al. published findings in the Journal of Sleep Research in 2002 on how brief naps affect overall alertness and cognitive performance. Sixteen healthy, young adult sleeper participants were limited to 5 hours of nocturnal sleep. During the day, subjects either took no nap, a brisk 10-second nap, a 30-second nap, or a 10-minute nap. Not surprisingly, there were not measurable improvements in subjective alertness, objective alertness, fatigue, vigor, or cognitive performance on a number of tests for both the 10-second and 30-second naps. In contrast, subjects who took 10-minute naps were significantly more alert and performed better than did the no-nap control group.
Researchers from the Department of Behavioral Sciences from Hiroshima University in Higashi-hiroshima, Japan found naps stopped at sleep stage 2 improved subjective awakeness, performance level, and confidence when asked to perform a number of cognitive tasks. They analyzed EEG readings for 7 young adults who had normal sleep-wake schedules prior to initiating daytime napping. Participants alternated nap and no-nap conditions weekly. To control for physiological resting, no-nap conditions consisted of resting in a semi-reclining chair but not sleeping. Researchers postulate that numerous napping benefits come from stage 1 and stage 2 sleep.
A followup study from the Hiroshima University using 10 healthy university students suggests 3 minutes of stage 2 sleep provide the most napping benefits. Student nighttime sleep was limited, and how long students spent in sleep stages was controlled for via EEG. Results indicate stage 2 sleep enhances daytime vigilance significantly more so than does stage 1 sleep.
In the No-nap condition, subjective mood and performance deteriorated, and Slow eye movements increased during mid-afternoon, suggesting that the post-lunch dip occurred. In contrast, subjective alertness and performance improved and slow eye movements rarely occurred in the S2-nap condition. Although subjective sleepiness and fatigue improved, performance deteriorated and slow eye movements increased in the S1-nap condition.
ABC news featured Rosalind Cartwright, chairman of the department of psychology at Rush University, and Dr. Alon Avidan, associate director of the sleep disorders program at UCLA, to discuss the best time to nap. Both advise napping 15-20 minutes–less time reduces benefits of napping (also indicated by Hiroshima University study) but more time risks entering deep sleep. Entering stage 3 or stage 4 sleep runs the risk of disrupting the body’s circadian rhythm, which might negatively impact nighttime sleeping. Also, waking from stage 3 or stage 4 sleep results in drowsiness:
The longer you nap, the more likely you are to wake up from deep sleep, leading you to feel confused and groggy. If you sleep [too late in the day], the tendency would be to get into the first deep sleep of the night from which you would wake groggy and grouchy.
So there you have it. As much as I love my sleep, occasionally I need to work nights. When I can’t get my 7-9 hours of nighttime sleep, I’ll just get the next best thing: 20 minute naps just after lunch.