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Why do sleep stages change?

Occasionally I'll sleep a solid 6-7 hours... Get up, look at my fitbit sleep stages, and then fall back asleep for another hour or two.

 

Sometimes after my 'first sleep' (6-7 hours - most of the night), my fitbit shows specific sleep stages. Say, 'deep sleep' from 10:45pm - 11:15pm... Or what have you. But when I check it later after my second shorter sleep, it COMPLETELY changed into a different pattern. 

 

Why is this the case? Why should a later sleep completely negate earlier calculations? It just makes me question the accuracy of the sleep tracker...  

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You're right; it does not make any sense that it should radically change sleep estimates from several hours earlier. I too question the legitimacy of this sometimes. 

 

For instance, last night it says I had a 32 minute block of time with straight deep sleep. Then, immediately following that it says I was awake for 11 minutes, which I don't see any way how that can be right. Awakening from deep sleep produces more sleep inertia than any other sleep stage, as it's the hardest one to awaken from. I know when I've been awakened (or more appropriately startled) out of this stage, as it is known to cause a lot of disorientation. I did not experience that last night (although at some point I did wake up abruptly out of a vivid dream, but that would have been from the REM stage).

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