iPhone battery health

I have typically been filling up my phone to 100% and using it till it gets to 1-5% because I have been told that is better for the battery. I am a light iPhone user, so it usually takes me 2 days to through a full battery.

Recently, I started using an app for sleep monitoring which requires me to plug in my phone every night. Am I accelerating the decline of my battery health by plugging in every night at 40-50% rather than filling it and letting it drain completely for each charge?