1to sleep (typically lying down); to be asleep.
毎晩十一時に寝るようにしています。
I try to go to bed at 11 every night.
赤ちゃんがぐっすり寝ている。
The baby is sleeping soundly.
飛行機で長時間寝るのは難しい。
It's hard to sleep for a long time on a plane.
2to go to bed; to lie in bed (prepare for sleep).
風邪気味だから早く寝ることにする。
I'm feeling slightly ill, so I'll go to bed early.
もう遅いから今日は寝るね。
It's late, so I'm going to bed for today.
3to lie down (not necessarily to sleep).
ちょっと横になって寝ると気分が良くなるよ。
Lying down for a bit will make you feel better.
床に寝て本を読むのは姿勢に悪い。
Reading a book lying on the floor is bad for your posture.
4to sleep with someone — a euphemism for having sexual intercourse.
彼と一度だけ寝てしまったと告白した。
She confessed that she slept with him just once.
5to lie flat (of hair, fibers, etc.), to be pressed down.
湿気で髪が寝てしまった。
The humidity made my hair lie flat.
6to lie idle (of funds, stock, etc.); to remain unused.
この口座にお金が寝ているから有効活用したい。
There is money idle in this account, so I want to make use of it.
7to ferment or rest (used of miso, soy sauce, etc.) — to mature during storage.
この味噌は冬の間じっくり寝てから出荷される。
This miso is allowed to rest through the winter before being shipped.