The different colours are designating good (green, quality >70%), OK (orange, quality 40% - 70%) or poor (red, quality <40%) sleep quality.
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Arrow indicators alongside the daily quality, indicate the change from the last recorded session and the improvement (arrow pointing up) or decline (arrow pointing down) in percentage points. The respective indicators alongside the average quality and duration values show how the currently viewed session values compare to the average ones. A dash sign, would indicate unchanged sleep quality.