noise

[cale]

A hypnic jerk, hypnagogic jerk, sleep start, sleep twitch or night start, is an involuntary twitch which occurs just as a person is beginning to fall asleep, often causing them to awaken suddenly for a moment. Physically, hypnic jerks resemble the "jump" experienced by a person when startled, often accompanied by a falling sensation. Hypnic jerks are associated with a rapid heartbeat, quickened breathing, sweat, and sometimes "a peculiar sensory feeling of 'shock' or 'falling into the void.'" A higher occurrence is reported in people with irregular sleep schedules. According to the American Academy of Sleep Medicine there are a wide range of potential causes, including anxiety, caffeine, stress and strenuous activities in the evening. - Wikipedia

I get these "feelings of falling into the void" a lot.