The ideal nap length, according to scientists, is 20 to 25 minutes. Any longer and you’ll fall into a deeper sleep cycle, which lasts for about 90 minutes. This means when you wake up you will experience “sleep inertia”, or grogginess.

A power nap, also known as a Stage 2 nap, is a short slumber of 20 minutes or less which terminates before the occurrence of deep slow-wave sleep (SWS), intended to quickly revitalize the napper.

Salvador Dalí, in his 50 Secrets of Magic Craftsmanship for aspiring painters, outlined tips for the one-second micro-nap, including sitting upright, key in hand and upturned plate beneath. Once asleep, the key will drop and the ensuing clang ensures instant revival. Albert Einstein deployed a similar technique, using a spoon or pencil instead of a key.

Exceptions to this include essential naps when sick, which are often longer because our bodies require more sleep when dealing with an illness. Also, fulfillment naps in children should not be limited to 20 minutes, as children have a higher sleep requirement than adults.

If you nap too late in the day it’s a bit like snacking before main meal, it just takes the edge off your sleep hunger at night.

理想的午睡时间是20-25分钟。睡得时间长了，容易进入深睡眠或快眼动睡眠阶段。这个阶段人体的肌张力是降低的，心跳呼吸不均匀，如果进入这些阶段并从此阶段醒来的话，很多时候会感觉全身无力、心慌不适，有的人甚至郁郁寡欢、无精打采。

有效午睡，也叫浅度睡眠，指睡眠时间在20分钟以内的午睡，在进入深度睡眠之前就会停止，可以使睡眠者快速恢复精力。

萨尔瓦多·达利为有志成为画家的人写过一本书《魔幻技艺的五十个秘密》，其中列出了一秒午后小睡的方法：笔直坐着，手里拿一把钥匙，下面放一个倒置的盘子，这样一旦睡着，钥匙就会落下，伴随着“叮”的一声，人就会很快醒过来。爱因斯坦也使用过相同的技巧，不过是用的汤匙或者铅笔来代替钥匙。

不过这种短时间午睡的情况也有例外，比如对于生病的人和儿童来说，可能需要更长时间的午睡才能让自己恢复精力。

入睡时间建议选择在一点到两点半之间，如果到了下午三四点后再午睡，则容易造成晚上睡不着觉，引起生物钟的混乱。