Really creepy to feel. I can remember that mostly, how unpleasant it was. I hated that. It was like 20 years ago, as a student. I think it was while preparing for, or after, tiring exams. Maybe 2 or 3 times only, for me.
It may be induced by a sort of stress we're not familiar coping with yet. Just a raw guess from what I experienced.
Are you preparing exams currently???
"Cyril" pronounced "see real". I code stuff.
"To study and not think is a waste. To think and not study is dangerous." -- Confucius