Please provide the definitions of each sleep stage, otherwise the statistics are useless.

Thank you! Wish Apple support would just list it out at simply:

Core sleep is the equivalent of light sleep and takes place in the first few hours of your sleep. It is the most essential sleep stage. 

Deep sleep is exactly what it sounds like. It is the phase of the sleep cycle where it is harder to wake someone up.

Deep sleep is the most important sleep stage because it lets your body release growth hormone and works to build and repair muscles, bones, and tissue. Plus, it helps keep your immune system functioning.

REM stands for rapid eye movement and occurs at intervals during the late part of the night. It is considered to be optional sleep that is less crucial.