Nap calculator

How the nap calculator works

Enter when you woke up, when you plan to go to bed and how long you can spare. The nap calculator takes off the minutes you need to fall asleep and picks a nap length from what is left:

• 10 to 20 minutes if you have at least 10 minutes of sleep time. This is the default power nap.
• 90 minutes only if you slept badly last night and have at least 90 minutes of sleep time.
• Nothing in between. If you have 45 minutes, it still says 20 and tells you not to use the rest.

The alarm time is the nap length plus your time to fall asleep. With the defaults that is 20 + 10, so set it for 30 minutes after you lie down.

Why 10 to 20 minutes is the best nap length

Brooks and Lack restricted 24 young adults to about 5 hours of sleep, then had them nap at 3 PM for exactly 5, 10, 20 or 30 minutes of sleep, or not at all.¹ The 5-minute nap did little. The 10-minute nap reduced sleepiness and fatigue and improved cognitive performance straight away, with some benefits lasting up to 155 minutes. The 20-minute nap helped too, but the gains only showed up about 35 minutes after waking. The 30-minute nap caused a dip in alertness first and then improvements.

That dip is sleep inertia. A review by Tassi and Muzet found that waking from deep (slow-wave) sleep causes more grogginess than waking from light sleep, and that sleep loss usually makes it worse because it increases deep sleep.² Without major sleep deprivation it rarely lasts more than 30 minutes. This is why the calculator never suggests a nap between 20 and 90 minutes.

When a 90-minute nap makes sense

NIOSH training material on shift work notes that waking after about 90 minutes, the predicted end of a sleep cycle, may mean less sleep inertia.³ A review by Lovato and Lack found that naps over 30 minutes can cause grogginess at first but then improve performance for longer than brief naps, and that longer prior wakefulness favors longer naps.⁴ So the 90-minute option is reserved for days when you are short on sleep and have the time.

The best time to nap

Early afternoon is the most favorable time for a nap, according to the Lovato and Lack review.⁴ Some people feel a slump in the mid-afternoon. Monk links this post-lunch dip to a rise in sleepiness with biological roots, and notes it can happen even if you skip lunch.⁷

The calculator sets your nap window at 6 to 8 hours after you woke up. For a 7 AM riser that is 1 to 3 PM. This is our rule of thumb, not a figure from a study. It moves with your wake time so it still works for late risers and night shifts.

The latest time to nap

The Sleep Foundation says experts typically recommend adults nap 8 or more hours before bedtime, which for most people means before 3 PM.⁸ The calculator treats this as a rule of thumb: your nap must end at least 8 hours before your planned bedtime, and the latest start is that time minus the alarm length. If your early-afternoon window runs past it, the window is cut short.

Coffee naps

Hayashi and colleagues gave 10 young adults 200 mg of caffeine before a 20-minute nap at 12:40 PM. It was the most effective of the options they tested for sleepiness and performance, with effects lasting the full hour after the nap.⁵ Reyner and Horne had 12 sleepy drivers take 200 mg of caffeine plus a short nap during a 30-minute break before a 2-hour afternoon drive in a simulator. Driving incidents fell to 9% of placebo levels, against 34% for caffeine alone.⁶ The calculator shows this option only for short naps.

Limits

• The nap studies were small, mostly in healthy young adults who were not regular nappers.
• Time to fall asleep varies. If you often lie awake longer, pick a bigger number.
• Needing a nap every day despite enough time in bed can be a sign of a sleep disorder. Talk to a doctor.

To plan your night, use the sleep calculator. For a dark room in daytime, see sleep masks.

Sources

1. Brooks A, Lack L. A brief afternoon nap following nocturnal sleep restriction: which nap duration is most recuperative? Sleep. 2006;29(6):831–840. doi:10.1093/sleep/29.6.831
2. Tassi P, Muzet A. Sleep inertia. Sleep Med Rev. 2000;4(4):341–353. doi:10.1053/smrv.2000.0098
3. NIOSH. Training for nurses on shift work and long work hours. Module 7: napping, an important fatigue countermeasure. Nap duration. cdc.gov
4. Lovato N, Lack L. The effects of napping on cognitive functioning. Prog Brain Res. 2010;185:155–166. doi:10.1016/B978-0-444-53702-7.00009-9
5. Hayashi M, Masuda A, Hori T. The alerting effects of caffeine, bright light and face washing after a short daytime nap. Clin Neurophysiol. 2003;114(12):2268–2278. doi:10.1016/S1388-2457(03)00255-4
6. Reyner LA, Horne JA. Suppression of sleepiness in drivers: combination of caffeine with a short nap. Psychophysiology. 1997;34(6):721–725. doi:10.1111/j.1469-8986.1997.tb02148.x
7. Monk TH. The post-lunch dip in performance. Clin Sports Med. 2005;24(2):e15–e23. doi:10.1016/j.csm.2004.12.002
8. Sleep Foundation. Napping: benefits and tips. sleepfoundation.org

General information, not medical advice.