A sleep bed is where you sleep. Where do you sleep? In a bed, probably.

This can refer to any kind of sleeping place, including a bed, futon, or even an animal's nest or den. You'll often see it in literature, in phrases like 寝床に入る (to get into bed). When you're talking about your bed in casual conversation, you'll probably just say ベッド.

This word uses the kun'yomi readings of the kanji. You didn't learn the second kun'yomi reading for 床. You take とこ and rendaku it into どこ. How do you remember this? Just think about your bed. Then wonder, "where (どこ) is it?"