寝てたの

were ... sleeping?

→ 寝る
phrase Rare #27,819
were ... sleeping? A casual question form of 寝ていたの, used to ask whether someone was sleeping or had slept. The subject is understood from context.
JA: まだ寝てたの?
EN: Were you still sleeping?

もう寝てたの?

Were you already asleep?

— 恋する惑星 (1994)

寝てたの

Were you sleeping?

— ふたりのベロニカ (1991)

どれぐらい寝てたの?

How long were you asleep?

いつも窓開けて寝てたの?

Did you always sleep with the window open?

トムったら、仕事中に寝てたのよ。

Tom was sleeping on the job.

寝てたの?

Were you sleeping?

トムは寝てたの?

Was Tom sleeping?