Start sleep time

You may be doing nothing wrong. The sleep start/stop detection on the Garmins is not perfect by any stretch. You can edit the start/stop times pretty easily though, so if you lie in bed for a while before falling asleep, or lounge and relax in bed for a while after you wake up (or wake up in the middle of the night and take a while to fall back asleep) you will find the Garmin's "guess" at what happened won't match your experience. Edit the start/stop times and take it from there.

Firstly, thanks for replying. I have tried editing the start time of sleep which is straightforward enough but I was surprised to find that all it did was extend the amount of time awake? So my overall nights sleep total still says only five hours instead of seven. I now wear my old Fitbit at night for more accurate sleep. Apart from this I’m pleased with the activity tracking for walking, have yet  to try the cycling tracking.

Yeah, sometimes what you've described (not changing the total sleep time) has occurred with my Venu, too. Other times it seems to accept my adjustments immediately, other times it takes some time for the Garmin Connect dataset to update itself, and other times it never does reflect the change. A bit of a pig-in-the-poke it seems.

I guess it comes down to how "scientific" we expect the Garmin sleep analysis to be. It's not terrible at all. It's sometimes not great. But directionally I still find it of interest and indicative of how well my might's sleep went - not how precise the individual data points were landed. Good luck with it!