I think it’s general anxiety from the accumulated “thinking fatigue” during the day. There is a correlation with periods where I lacked a clear plan and spent too much time of the day in my thought planning the future. The process at sleep then is something like I feel tense -> I become hypersensible -> I notice each heartbeat accelerating -> maybe something is wrong -> ... Sometimes it was more sudden like waking up panicked in the middle of the night. Meditation helped because now I notice when I loose myself too much in my thought. The effect is not immediate but it makes a difference at the end of the day.
