Hello Raissa, 呢 is used at the end of a declarative sentence， to indicate a continuous state. It is often used with the word "正，在，正在". So, you can say either one: 1) 他在睡觉。 2) 他正睡觉。 3) 他正在睡觉。 4) 他睡觉呢。 They all have the same meaning. Hope they help, Tere