Of course there are going to be periods of slight movements that appear as noise while sleeping. But the device should be smart enough to realize when there has

Of course there are going to be periods of slight movements that appear as noise while sleeping. But the device should be smart enough to realize when there has been a very long (see 8 hours) period of inactivity. No one is staying still for that long while they sleep so the device should just assume any data collected during those 8 hours of inactivity was just the device sitting on a bedside table and that the data should be discarded. Additionally, ask the user if they want to record sleep data and let them choose the hours they typically sleep. Using that can greatly help save battery life during the night by just having the device be more liberal in its use of a low power mode when long periods of noise-level activity is detected. Use the general sleep window to put the device into a mode where it listens for the slight, noise-level movements. During the day, those movements aren't going to matter as much compared to the night so the window is important in clearly defining when they should count more.