Keys that significantly improved sleep (none are a secret, but it was cool to see the actual Garmin data back it up):

1) Eat well, exercise and lose weight. If your watch measures HRV, this is the biggest thing that improved both my heart health (while sleeping) and sleep quality).

2) routine

3) more than 6 hours of sleep

Dumb stuff like shut off the LED back lights (don't look at your phone) an hour before sleep actually help a ton too, but that's tough for me.