Actually - just a heads up, having more sleep before an event that your body considers to be important is not necassarily a good thing.
Many people may not get enough sleep before the UMAT due to "nervousness" - but that should not affect their tiredness the next day too much, the initial morning 'hangover' and weay eyes fade off by the time you get ready, and a quick 15 minute nap (nothing more!!!!) in the car on your way will cleanly wipe away any fatigue, hence the amount of sleep before one day will really not make any difference at all - I would nto recommend this lol, but I honestly even dare to think that an all-nighter before the UMAT may not impact your performance as much as you think it would - your body prepares itself psychologically for these event
The problem arises when there is CHRONIC sleep deprivation ORRR too much sleep - not getting enough sleep during the week leading to the UMAT will definitely cause fatigue and tiredness during the exam, AND getting more sleep than normal before the exam will actually also increase your 'sleep hangover' duration and can also make you more tired.
So basically - go to sleep whenever you want and wake up at your alarm (for the morning sessions) and for the later sessions, jump out of bed whenever you naturally regain consiousness, as going 'back' to sleep never helps (unless u go back to sleep long enough to resume REM cycles - but that takes 90 mins atleast
soooo TAKE HOME MESSAGE
having a morning session won't and shouln't harm your concentration as a one-off shortage in sleep will not actually cauase you to be that tired, (in fact in some cases, in the short run it can increase awareness, but that is somewhat different - dw about it)
so getitng a little less sleep before the UMAT is not the end of the world.... it shouldn't hurt you
if anything, having an afternoon session and unecassarily sleeping in and stacking over 8 hours of sleep can cause you to be more tired and "hungover" than someone who wakes up with less than optimal sleep.