Make it dark

Light and darkness distinguish day from night, which is why insomniacs are offered light therapy treatment (exposure to bright light at certain times of the day) to help them associate dark with sleep time.

If you want a good night's sleep, you need complete darkness, because your eyelids alone aren't enough to protect your eyes from any light. Close your curtains and blinds, remove any electrical equipment with phosphorescent displays, and if you have to get up during the night, avoid turning on bright lights.