On Galaxy Nexus and few Motorola devices, you may be experiencing an issue that the sleep graphs turns wild at a specific hour during all tracking.
From various error reports we know that the accelerometer sensor implementation on some firmwares has issues. Over the time the accelerometer may get into a broken state and it stops sending data. The only way we know to resolve this issue is to do a reboot. In all cases so far, reboot did solve this issue.
To test whether the accelerometer is working, you can use menu > test sensors or some 3rd party app such as Sensor List.
The red block indicates that something went wrong with tracking at that time. We did not get the expected data from accelerometer. Maybe Your phone has restarted or if You are using smartwatch, we could have lost connection to it.