If I don't get at least six hours for several nights, my mental acuity wanes
The demands of law school become less and less consistent over time. I had a pretty regular schedule during 1L fall, but after that it became more sporadic. Sometimes I average 4 hours of sleep a night over the course of a week. Some months I get 6 or 7+ almost every night. I probably average 5 hours of a sleep during the two weeks leading up to finals, and I try to sleep 7 or 8 hours a night before my actual tests.