I've been stuck on <30 minutes of deep sleep for as long as I've had an Apple watch and probably much longer. Some observations I recently made:

- Japan has these drinks with 200mg of GABA, I don't know if it was the 20,000 steps we were doing every day or the drinks but on the days I had it deep sleep was often (not always) closer to an hour. Strangely enough GABA is illegal here in NZ so I'm trying L-Theanine with less obvious effects.

- Exercise (even 2h+ moderate) doesn't have any effect, adjusting caffeine doesn't seem to either

- A warm shower immediately before bed strangely did seem to have a considerable although not perfectly consistent effect

- Just got put on Concerta for ADHD and although im getting less sleep i'm also getting 40+ minutes of deep sleep. After recently doing research into GABA + Dopamine + etc it seems to be a bit of a brain circuit thing for me.