My goal is 7 hours of sleep but I accept 4. When I was in the Army we were told that we were "good to go" if we had 4.
I also have a 20-minute guided meditation that I use for power naps during the day, and I have found that extremely helpful. Finding the right guided meditation may take some trial and error, but I've been using mine effectively now since the '90s (I had the cassette transferred to digital). I just Googled "power nap guided meditation" and got a bunch of hits.