The Periscope

So how do the ones under the bed know they can come out? Well, it is as simple as looking through a periscope. As you close your eyes, the ones under the bed raise their periscope to check if you're asleep. If your back is towards it, it will stay up for as long as the ones under the bed wish to observe you. However, should you turn to face it, the periscope will retract faster than the eye can see.

There is a way to spot it, however. Simply hide under your blanket and/or barely open your eyes when facing the edge of the bed. There, you will catch a glimpse of a dull grey periscope. You won't see your reflection, nor the eye of the one gazing up from below. Though as soon as your vision is sensed, the periscope will retract.

Fear not, the ones viewing you have no malintention. They are just watching you sleep. Though the periscope will only emerge if you have at least one side of the bed not blockaded by walls or furniture. It will also only come out at night. Even if you make the room as dark as possible during the day and fall asleep, neither they nor any of the others that lurk under the bed will come out to observe you.