When he isn't OT or sick how many hrs of sleep does he average in a 24 hr period? Since he's probably going to be caught up after a great night and nap now and is likely no longer OT
then I'd aim for a total A time of 24 hrs minus the average # of hrs he gets. So, if he averages 14 hrs of sleep in a 24 hr period, I'd aim for a total A time of 10 hrs that day - which today would be 4 hr 40 since morning A was 4 hr 20. Does that make sense? But again, that's just what I'd do as that's what seems to make the most sense to me. Feel free to disregard
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And, if he usually takes 10 min to fall asleep for BT then yes I'd PD 10 min earlier than the A time you're going for.