1. people who are sleeping
2. railroad ties; wooden or concrete beams that support railway tracks
Usage: British
3. things or people that achieve unexpected success after being overlooked
Usage: informal

Examples

• The heavy sleepers didn’t wake up during the thunderstorm.
• Workers replaced the old wooden sleepers with concrete ones.
• That small-budget film became one of the year’s biggest sleepers.
• The baby wore cozy fleece sleepers to bed.
• Railway maintenance crews inspect the sleepers regularly for damage.
• Several sleepers at the campsite were disturbed by the noise.
• The stock market had several sleepers that surprised investors.