It's because you're tired after getting up so early after only a few hours of sleep, not because you had a wet dream.
Wet dreams are natural, every boy has them, and they always stop. Obviously they're embarassing so if you find yourself waking up keep some tissues by the side of your bed and ejaculate into them so your pyjamas and sheets don't get wet. If you're too late and you've already ejaculated then wipe it up as soon as you possibly can, put some tissue on the sheets to absorb it and change your pyjamas. Clean under your foreskin after you've had a wet dream and consider sleeping without pyjamas (so there's less to clean if you have a wet dream).
Not sure of your age but many times you have a wet dream because your body is trying to regulate the amount of spree it should produce. You have a couple options. 1 you can start masturbating regularly or abstain and allow your body to figure things out.