The night was commonly divided into three watches (see Lam. 2:19; 1 Sam. 11:11). Doubtless David found it hard to sleep when pursued, yet he was sufficiently tranquil to spend his sleepless hours in meditation.
Upon my bed.
That is, during the night. At such a time the difficulties of the day tend to be magnified. David doubtless spent much time thinking about God during the anxious nights in the wilderness. We would do well to turn our thoughts toward God during our sleepless hours.