Copied from my answer here.

• Take melatonin at the appropriate time and dose. It's cheap and legal in the U.S., but most products have way too much. https://slatestarcodex.com/2018/07/10/melatonin-much-more-than-you-wanted-to-know/ most insomnia drugs are not much more effective than this.
• Avoid light at night, especially blue light. Light inhibits natural melatonin production, which interferes with your circadian rhythms.
  • If you can't darken your room completely, you can use a sleep mask instead. Get the kind with cups (like opaque swim goggles) instead of the kind that puts pressure on your eyes.
  • Use f.lux on your personal devices to reduce blue light after sunset or use one of the similar built-in features of your OS. Windows 10 has the new "Night Light" setting, macOS and iOS have "night shift" mode. Newer Samsung phones have a "blue light filter" setting. These options vary in quality and may have configurable intensity. More intense is more effective and it's surprising how much you get used to it.
• Falling asleep is a common failure mode of certain types of meditation practice. You can use this to your advantage when suffering from insomnia in bed. Even beginners fail to meditate this way accidentally, so it's not particularly difficult to do on purpose. Focus your attention on the sensation of breathing or on the ringing in your ears. When you notice you are lost in thought, refocus your attention. But when you notice the dreaming arise without directed effort, dive in and let them take you. It works for me anyway. If not, at least you got your meditation in today. [...]
• Track your sleep quality.
  • You can get smartphone apps that purport to do this using the phone's sensors. Some fitness trackers or smartwatches also have this function built in or available as an app. Accuracy varies.
  • You may have sleep apnea. Talk to your doctor about doing a sleep study to diagnose possible issues and treatments. Some people do much better on a CPAP, but there are many other treatment options.
• Avoid eating late at night. This can cause indigestion, which can keep you awake.
  • if you suffer from heartburn, sleep on your left side to contain it better, because your esophagus attaches to your stomach on the right side (unless you're one of those rare people with backwards internal organs).
• Exercise regularly. I'm not sure why this helps, but it seems to. Perhaps mental fatigue doesn't always line up with physical fatigue unless you actually make some effort physically during the day.