Working hours visual indicator
In other calendar software (e.g. Outlook), users are able to specify "working hours" which are then coloured differently (with a lighter shade) in the day or week view.
This helps with visual decluttering - for most, 1:00 AM isn't as important as 11:00AM, for example, so it makes sense to display those differently.

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DaMoIsHere commented
Thought of the same thing. I currently have a short "Wakeup" and "Bedtime" event created on every day to more easily see which part of the day I can actually put things in.
However this really clutters up the calendar and especially the Android widget. Having the option to e.g. put a darker background color from 10p.m. to 6a.m. to indicate nighttime would be great.
The times and days where they apply should be adjustable. Perhaps one could even set them to sync with local sunrise/sunset times.
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Marcos Dias Alves commented
OK, but it MUST be optional AND configurable by the user.
Fastmail has deployed this recently and I can not use the calendar anymore due to the greyed background on what "they" consider as non-working hours; the calendar view is a mess now. Fastmail does not allow us to turn it off; in Outlook you can configure the working hours as you want.