There is a general recommendation that you should essentially book your exam at a time you are most alert and fresh.
The same can perhaps also be said to be true of when you STUDY as well.
It's completely understandable that many test-takers have other commitments going on during the day and can only squeeze an hour or so at night before bed...but it may make your studies more effective and efficient to "book" a better time. Instead of forcing yourself to stay up an extra hour when you're completely frazzled, why not wake up an hour earlier to do those triangle questions? Maybe squeeze an RC passage or two during that green tea break in the afternoon - yes, I'm a green tea drinker!