What it is for
Planning a nap is mostly about duration: too short will not refresh you, too long can leave you groggy. This calculator adds the time you take to fall asleep to the nap length you choose and shows the wake-up time. Formula: wake-up time = start time + minutes to fall asleep + nap length.
The four lengths
A 20-minute power nap tends to stay in the lighter stages of sleep, and general sleep-medicine guidance suggests naps of about 10 to 20 minutes to reduce the chance of slipping into deep sleep. A 30-minute nap may already include some deeper sleep. A 60-minute nap tends to include deep sleep. A 90-minute full cycle approximates the average length of one sleep cycle, and waking at its end is usually easier.
Sleep inertia
Sleep inertia is the grogginess and dip in performance right after waking, more frequent when you wake from deep sleep. That is why short naps or full cycles are the two most cited strategies. Classic studies, such as NASA's work with long-haul flight crews, found that short naps can improve alertness and performance, though individual effects vary.
Worked example
Lying down at 14:00 for a 20-minute power nap and taking 10 minutes to fall asleep: set the alarm for 14:30. With the 90-minute nap and the same 10 minutes: 15:40.
Limits
The information is general and promises no particular effect. Long naps or naps late in the day can make night sleep harder for some people. If you feel the need to nap many times a day, or can fall asleep in dangerous situations, talk to a health professional.
Frequently asked questions
What is the best nap length?
It depends on your goal. To recharge without grogginess, 10 to 20 minute naps are most often suggested; 90 minutes allows a full cycle.
What is sleep inertia?
It is the temporary grogginess after waking, most common when waking from deep sleep. It usually fades within minutes.
Why count the time to fall asleep?
Because the 20 minutes of napping only start once you are asleep. If you take 10 minutes to drift off, the alarm should ring 30 minutes after you lie down.
Can I nap in the evening?
Late naps can interfere with night sleep for some people. Many guidelines suggest avoiding naps close to bedtime.