Why do we enter REM during naps?

I never quite understood why we enter REM sleep during afternoon naps, people have said the best time to WILD to them are the afternoon naps, Doesn’t it take at least 90 minutes to enter REM? I don’t understand.

There are 2 types of sleep:

  1. The heavy sleep, which consists of several sleep cycles, which vary from NREM 1,2,3 sleep and REM-Sleep. Such sleep normally lasts for about 8 hours.
  2. The light sleep or afternoon nap, which consists of very light sleep (Mostly REM-Sleep, which may be accompanied by NREM 1 sleep or, rarely, if you’re very tired - NREM 2 sleep. Afternoon naps may last anywhere from 10-20 minutes to 1-2 (more if you’re tired) hours.

In the first scenario, which usually happens in the evening, you go trough a lot of NREM sleep cycles. During these cycles you do not usually dream (although there are reports that one may have non-vivid, hallucination like dreams in NREM-3 stage). At the start of this cycle, you first enter NREM-1 sleep. This sleep usually lasts for 5-10 minutes and is usually described as a half asleep - half awake state. You may have very quick body movements during this stage, which may wake you up, known as hypnic jerks. After this stage, you gradually pass down to heavier NREM sleep stages: 2 and 3 (There used to be a fourth, but it was merged into the third). During these stages you lose consciousness and your body is regaining it’s energy, which was spent during the day. After around 90 minutes of such NREM sleep, comes the first REM sleep. It last for a few minutes. During this REM sleep you will dream, although at such early sleep the dreams will be not as vivid and hard to recall. After this, the whole cycle repeats for around 5 times, followed by brief awakenings.

Now you may realise, that during this sleep, it’s hard, if not impossible to do a succesfull WILD attempt. If you try to do a WILD during evening sleep, you will have to wait at least 90 minutes of NREM sleep, until the REM comes.

The second scenario is afternoon naps. When you go asleep during afternoon, your body will not go trough the heavy NREM stages, as the purpose of naps is just to briefly relax, and energize your body. During naps, you will enter NREM-1 sleep, which will be immeadeately followed by REM-Sleep. As NREM-1 sleep only lasts for a few minutes, you will not have to wait 90 minutes as in the previous case. Therefore, WILD is available option here.

So the best time to do WILD is:

  1. After 4,5-6 hours of evening sleep (As that’s when the longer REM cycle occurs)
  2. During naps (As it mostly consists of REM sleep)

Here is an image, which shows an average sleep of a human:

Thanks for the reply! I understand now.