because that 'heal' button comes up when a unit has suffered damage. if i hit fortify/sleep, will they heal anyway?

Yes, units will heal when fortified. The distinction between the two controls is that "Heal" wakes the unit up once it is completely healed, whereas it stays fortified if you use the "fortify" command.

thx man

also how come at times you see sleep, and other times you see fortify?

Not all units can fortify. Fortify means they get a defensive bonus if they are attacked. Things like workers, settlers, spys, some of the horse units, etc. just sleep until you wake them up.

Does sleep give the unit any bonuses/losses?

Does sleep give the unit any bonuses/losses?

Units that can't fortify, don't receive any defensive bonuses either. No terrain bonus, no fortify bonus.

...But they still receive bonus if attacked from behind a river.

Also note that if you use the Heal button the unit still fortifies if it is a unit that can.

The purpose of the heal button is that the unit becomes active again as soon as it is at its full strength. So you don't risk to 'forget' units somewhere, or check every turn if there are units sleeping somewhere that are ready to get back to the frontline.

Ok, I have a question too - sometimes my unit still have to wait one more turn to full heal, and it's strenght is 1.0 (let's say it's a Scout) instead of 1. How big is this difference? Can I just use my Scout turn earlier when it's health is like 99% or does it work different?
Anybody knows?

That just means the strength is something like 0.97 and it's rounding up. It's not a huge difference and I'll use Scouts in that situation all of the time. If you want him fully healed then you'll have to rest that 1 extra turn.