I've taken to not using a screen within about an hour of going to bed, and never using the phone in the bedroom (because of association), and my sleep is much better - I fall asleep quicker, and study asleep longer. I tested the opposite on myself previously (intentionally using the phone in bed right up to lights out), and took longer to get to sleep, and woke up around 4-5am feeling not very rested.

Smartphones are the devil.