before i share the magic formulas heres some terms ill be using:

Sleep: your sleep meter. 100% sleep is full, sleep at 0% is empty, and 50% sleep is half full. might be a little weird to say 100% is fully awake but whatever.

rng: it stands for random number generator but ill be using it to say randomness. an "rng call" is one instance of getting a random value.

ok? lets start

When you load into the game from the main menu, aka continuing a save:
If sleep <= 45%, there is a 30% chance for it to be night

When you return from a mission:
If sleep > 75%, it cannot be night.
If sleep > 35% (and <= 75%), there is a 9% chance for it to be night
If sleep < 35%, there is a 56% chance for it to be night

now if you have done a good amount of programming, you might have noticed something. there is no case for when sleep = 35%. what this means:
If sleep = 35%, there is a 20% chance for it to be night

the statements leading to these probabilities are a bit weird, and out of respect I will not be sharing the (decompiled) code but if you're curious enough it'll be easy enough to figure out ;)

so you want to return from a mission but sleep <= 75% (75% being the amount required for it to never be night)?

energy drinks!

they can be found in the cafeteria or lying on boxes/crates/whatever. when you drink an energy drink, theres some math to how much sleep you get

If sleep < 10%, sleep is set to 10%
then
a random amount of sleep from 0% to 20% is gained

with average luck, itll take 7 energy drinks to have > 75% sleep! with good luck, itll only take 4 energy drink!

and thats it. get lucky with energy drink rng rolls ;P