I cannot remember where I got the following, must have been some scientific research on brain acitivity about sleep.
Anyway, I recall that in approximately every two hours of sleep, you lose about 20% consciousness. So for the first 2 hours (1-2), you're in deep sleep. In the next 2 hours (3-4), you're 80% subconscious probably still snoring away. Then for the next 2 hours (5-6) you're 60% subconscious or 40% conscious. Brain activity starts. You can imagine after 6 hours your brain in active gear though your eyes may still be closed. After 10 hours you toss and turn and wake up with a headache, oversleep.
So if you can get at least 2 to 4 hours of deep sleep, at whatever time of the day suitable according to your own schedule (e.g. if you are a nightwatchman), you can follow it up with decent days work plus Yoga.
The same research said that elderly people's cycle is shorter than 2 hours and young babies and kids longer than 2 hours. So you'd notice kids sleeping long whilst notice many elderly people slep a few hours though they may replenish their energy with more frequent in-between naps.
Anyway, that's my non-scientific answer, but Amrit Vela is the time when you can taste it most sweet, and when you're fresh after best sleep, as during the time when you are easy to float into dreaming mode, you immediately counteract it with your Amrit Vela. Mind you, I cannot always follow this sleep pattern, must be sanskars