The day cycle is a period of 18 hours in game (or 13.5 minutes real time; one in-game hour is 45 seconds) from 6:00AM to 12:00AM (Midnight). Past midnight, the player's character becomes tired and in need of sleep. However, The Player can stay awake until 2:00AM, after which their character will pass out.
If the character passes out they will wake up in bed, having been dragged there by either the town's doctor, Harvey, Linus, a JojaMart member or JojaMart doctor. In each case some of The Player's money will be lost (the details of this transaction will appear as a letter in the player's letter box). An exception is if the player was lucky enough to pass out inside the house.
It is considered nighttime after 6:00PM, which makes it possible to catch night time fish. However, it doesn't become dark until 8:00PM.
Sleep will restore the player's Energy and Health to full if they go to bed before 12:00AM and are not exhausted, and to half if they pass out or are exhausted. The longer past midnight the player stays awake, the less energy they wake up with (regardless of amount before sleep), with about 50% if they sleep right before 2:00AM.