sleep like a log

This is another way of saying you slept really well. People feel like they had good sleep when they obtain deep sleep. Deep Sleep is one of the 4 sleep cycles and some think it is one of the most important. Since logs do not move by themselves once placed on the ground, to sleep like a log is sleeping so good that you do not move.

A similar saying to "sleep like a log" is "slept like a baby". Both are ways of saying you slept undisturbed throughout the night.

The opposite is when someone says they tossed and turned all night. This is not sleeping like a log. You are moving all over the place which means you are not getting quality sleep. It is likely you are getting light sleep vs deep sleep. This is when people wake up still feeling tired.