how can i sleep early at night?

i've been taking melatonin to help me sleep, and it does help, but i keep having nightmares and wake up between 2-4 am. also, lately, i feel like it's taking too long to kick in. i was planning to take two pills at once, each tablet containing 3mg, but i don't want to risk it because of medical issues. what do people usually do to sleep early every night?