Staying up late; remaining awake past one's usual bedtime (for study, work, entertainment, etc.).
明日は試験だから夜更かしはやめなさい。
You should stop staying up late — the exam is tomorrow.
週末はつい夜更かしして映画を観てしまう。
On weekends I end up staying up late watching movies.
子どもが夜更かしすると翌日機嫌が悪くなることが多い。
When children stay up late, they are often in a bad mood the next day.