Post the images of your HR graph and the sleep diagram from GC Web (not GC Mobile!) - both for the same night. When taking the HR graph, select first the period of your sleep with the mouse - the graph will then reload with just the selected period, and the details will be better visible. At the sleep diagram, click the gray button Movement under the diagram, so that we can see the movement curve too.
Also tell what Garmin device you use, and what firmware version it runs (find it in watch's Menu » Settings » About)
Having this info, we canperhapstell you why the device did not recognize your sleep correctly. If not, then a visit at a specialist with adequate equipment would tell you better.
You also may want to read for example this andthisdocuments explaining how the sleep, and its stages, are being detected by fitness devices, and you will see what factors and caveats are involved with using this technology.
