Fun game play :) Needs to move to mobile! (Left jump, and right switch). You need to 'design' platform groups and keep them coming randomly.
It's nice to add coins BUT they didn't seem to have any purpose - why would I want to collect them? Find a reason :) Either buy boosts, higher score, etc...
The character is cute, but seems to be 'different' from the background. Try to have a more satisfying feeling when switching day and night.
Would be fun if later in the game something happens that breaks the mechanics - a super speed run with lots of coins, an a ability to double jump, a 'forced' day or night, a better score if you switch less between day or night, etc...
I enjoyed it very much.