Sleeper (noun)

1. A person who is asleep.
2. A piece of furniture, such as a sofa or bed, that can be converted into a bed for sleeping.
3. A person or thing that is not initially successful or popular but later becomes so.

Origin:

Mid 17th century (as a noun denoting one who sleeps): from sleep + -er.

Examples:

1. The baby was such a good sleeper that he only woke up once during the night.
2. He was so tired that he fell asleep on the sleeper couch.
3. The movie was considered a sleeper hit as it slowly gained popularity over time.
4. The company's new product was a sleeper in the market, it took a while to gain traction but eventually it became a best seller.
5. He was a sleeper agent, a spy who had been planted in the organization years ago and was only activated when needed.