Sleep monitor - Meaning of colours

[Prav153]

I am trying to make sense of my sleep graph esp the colour coding. As you can see in the picture, left side is all grey and right side full colour but otherwise pattern is same. 

I would like to know what do different colours signify.

 Many thanks in advance

 

 

[Dannyboy2018 · Accepted solution]

Light,Deep,Rem

[Shazzkid · Accepted solution]

Don't forget "awake"

[pjdjphattunes · Accepted solution]

If you tap the grey the details will appear. It's detected 2 sleep sessions.