This year i tried that things fall in right place, i started sleeping early (by 10.30-11) ,3 months back from exam (with few exceptions).
Taking this habit of sleeping late to the last weeks before exam is a perfect disaster.
We cannot expect to change circadian rhythms of our body in one or few nights, even if we manage to get up on D day , but our mind will not perform with its full efficiency.
so best is to make most out of the remaining time before exam- keep this thing at high priority.
(p.s.- try to keep iris constant - it would male hard to think and eventually make you fall asleep)