I’m surprised tho’ that in 81 posts nobody has mentioned Circadian rhythms !
It seems that like most things, sleep is cyclical and we return to a very light sleep approximately every 2 hours.
If sleep is limited then, you’ll find it easier to wake alert after around 2,4,6 8 hours.
About 38 years ago I had a light haulage firm and I used to say "There are 168 hours in a week - I can USE 120 of them ! " which leaves 48 hours for eating, personal hygiene, intimacy and everything else (mainly on a Sunday).
Strong Black coffee helps, as do caffeine piils, but cold fresh air and loud radio helps too.
When sleep time comes, a good slug of distilled spirit will put you to sleep fairly quick too.
A few years ago, prostate problems meant I had to get up around 6 times a night, to pee. which reduced sleep to maybe 3 hours, spasmodic. - The operation cured that to an extent.
Poor sleep can be a great help if you are under great stress - because you can get up and do your research, answer letters and get quality thinking time when everyone else is asleep.
Don’t worry about it too much - your body knows when it’s knackered and sleep will just come !
I note someone said a hard mattress was a difficult place to sleep - I actually find the harder the better ! Takes a week or two to get used to it - then you wouldn’t go back to “soft” for all thetea in china !