One. The phone

Two separate things happen with the phone at night and only one of them is about light.

The light matters, though it matters most when a bright screen is close to your face for a long stretch rather than for a glance. The bigger problem is usually the content. A phone is an engagement machine, and something that holds your attention holds your alertness with it. The argument in the comments, the message you should not have opened, the video that led to the next one.

There is also a quieter effect. If the phone is what you reach for when you cannot sleep, then it becomes part of the bed, and the bed picks up meaning as we said in lesson two.

The simplest version, and it is not subtle: charge it in another room. Buy a clock if you need an alarm, and turn its face to the wall.