Dark Period

Dark Period is the practice of not letting any blue or green light enter your eyes, and it is usually practiced from before the first core until the end of the next sleep at least 7h after the start.

In schedules like M1, this means DP is only done before the core, because the core end is already >7h after the start of DP.

In schedules like M2, this means DP lasts from before the core until the end of N1.

Here's a way to determine when to end:

1. Find when to start: Ideally, 2h before the desired core.
2. Add 7h to the start time
3. Look further in time until any sleep phase ends
4. This is where you end DP.

Along with not letting any non-red light enter, no food should be consumed during DP as that can also worsen sleep quality through digestion, and it can misalign the circadian, sometimes moreso than blue light.

How to block nonred light

The polyphasic community has established itself as a large buyer of red laser goggles. This has also been recommended by many doctors, even those not involved in polysleep, as one of the most effective ways to get better sleep.

Using a phone's eye comfort shield is not enough.

The recommended goggles have a red tint and they should have a visible light transmission of 30% or more for comfort. If no VLT is specified, they're usually not below 30% or it's usually going to be visible on the product images as nearly black.

Why DP?

Without DP, the following things happen:

• You delete any SWS in the first sleep cycle of your core
• You mess up your circadian
• You decrease sleep quality for any sleeps that would've been during DP

In short, your sleep quality suffers greatly, leading to a higher possibility of being unable to adapt (cant-adapt).

Length before the core

For most, 2h is the ideal start time, but if you get tired much quicker than that even without sleep debt, reducing that to 1h can be a better option. Less than an hour is highly unlikely to be sufficient for SWS gain however.