I read your original topic and have the same issue. I have my nap replaced by my regular night time sleep a couple of times now and it messes up my sleep score. I have to manually change my sleep time to my original sleep time. This improves the sleep score but I have noticed that it's not the same as the original sleep score.

I checked Garmin support website and it says that i have to put my watch in sleep mode and then it will record naps in addition to sleep time. I haven't tested this.

support.garmin.com/.../

The problem is that sometimes I don't plan my afternoon naps. I could be in bed reading and might fall asleep. So I don't have a chance to put the watch in sleep mode. Garmin can clearly detect naps which is great. But there should be an option to add this nap without having to put my watch in sleep mode.

So, Garmin already has what I'm suggesting above? Only what is missing is to not replace regular sleep with a nap. Good developer can implement during lunch break

Btw, suggestion to manually put watch to sleep mode is ridiculous. The whole point is to detect nap/sleep. I know, wrong detection could happen but it's easier to delete that then when feel sleepy remember to switch watch to sleep mode.

How does a nap started manually via "sleep mode" affect important data like training readiness, recovery time and so on ? Are these data also additionally calculated during the recording of the additional sleep ?

So if I need 10 hours of recovery time after the night sleep, and I take an extra sleep, will the recovery time be adjusted by this extra sleep ( manually started sleep mode) ?

The pure graph with my sleeptimes is secondary for me. For me, the additional fitness data resulting from the extra sleep is more important. 

From the linked support articles, I can't see how a nap affects these Fitness-data.

Personally, I think the body battery and exercise readiness calculations just can't do it when there are two sleep data sets. And Garmin has no solution for that. Therefore, there may only be one sleep data and these are used for the calculation. Unfortunately, then totally wrong if you slept 2 hours in the afternoon, but 8 hours at night. The 8 hours are overwritten and the 2 hours are used for the calculation.

Well, then Garmin should use only night sleep or whichever is longer, night sleep or a nap. Or make algorithm just a bit more complicated and add nap into calculation. Similarly how resting can slightly increase body battery, nap can just increase more.

Today, I took a nap and put my watch in sleep mode before falling asleep. And even after doing this, it has replaced my last night sleep and given me a zero sleep score. So as far as I can tell even putting the watch in sleep mode manually is not working to avoid this issue.