I'm trying hardcoe mode for my first playthrough, which is a blast!

Sometimes when sleeping, however, I don't regain any health.

Is it because you can only sleep a certain number of hours in hardcoe mode before it stops raising your health? I thought maybe my thirst level was too high, but that wasn't it.

On hardcoe mode only seeing the doctor, stimpacks or using a doctors bag will heal

Actually, sometimes sleeping heals me.

Sometimes it doesn't. after sleeping a few times in 1 hour increments, the 3rd or 4th hour doesn't seem to heal HP anymore.

Drinking water will also heal. Continuously drink non-irriated, if available, by holding down E to heal yourself. No longer hurt, no longer lacking hydration.

Yes, water certainly helps.

I don't have a problem with healing, etc., I've been doing ok in the game and having a great time.

I just want to understand the mechanics of how this works and why sometimes sleeping heals and some times it doesn't.

Sleeping does not heal. Probably what you are noticing is that you ate something before going to bed. You still get the benefits from that while sleeping.

So, drinking a Nuka Cola gives you +2 HP/s for 15 sec. If you drink, then sleep before that 15s is over, you will wake with more hitpoints than you started sleeping.
Same regards when it comes to food.

Seems pretty clear. Sleeping (by itself) does not heal a single HP in hardcoe.