At in-game twelve o'clock am, your player will become tired. If you are still out of your home at two a.m., you will stop whatever you are doing and fall asleep. You wake up the next morning in your bed, with a note in your mailbox that either a Joja Co team carried you home, Linus found you unconsious, or you were taken to Dr Harvey after being found by a neighbor (if youre not married to him). You lose up to and including 1000 dollars in medical fees or from someone going through your pockets depending on how youre found.
Sleeping affects the rate at which time progresses. When asleep an hour becomes 100 minutes which affects processing times. Going to bed early means things will process a little faster in kegs, casks, and preserves jars, but may mean missing out on in game events such as heart events or holidays like the Dance of the Moonlight Jellies.
The world becomes dark at nighttime. If a light source disappears at nighttime, nearby NPCs will run away. The inside of houses will need to be lit as well as any spaces outside on the farm that you may still be working on once the sun goes doown.
There are four seasons each year consisting of 28 days each: Spring, summer, fall, and winter. Each season starts on a Monday and ends on Sunday, with 4 weeks total making a year 112 days total. On the first day of each season, any crops left planted the previous season shrivel up and need to be cleared away with a scyth before any more can be planted (with the exception of cross season plants like coffee beans which grow spring and summer, corn which grows summer and fall, wheat which grows summer and fall, ancient fruit which takes a season to grow and continues to produce until winter, and tea leaves which can be harvested the last week of each season until the start of winter). No crops can be planted in winter with the exception of Winter Seeds which produce winter foragable plants (winter root, snow yam, crystal fruit, crocus, and holly).