# sleeper /ˈsɫipɝ/ (US) · /slˈiːpɐ/ (UK) Source: https://englishreference.com/word/sleeper Updated: 2026-08-08 ## Senses ### 1. a person sleeping noun · CEFR A2 a person who is sleeping, often described by how well they sleep. For teachers: a person who is in a state of sleep, typically categorized by the quality or depth of their rest. - I am a very heavy sleeper, so the loud thunderstorm didn't wake me up. - As a notoriously light sleeper, she found it impossible to rest in the city center due to the constant hum of traffic. ### 2. railway car noun · CEFR B1 a train carriage that has beds for passengers to sleep in during a long journey. For teachers: a railroad car equipped with berths or compartments for passengers to sleep in. - We booked a sleeper for the overnight trip from London to Edinburgh. - Traveling by sleeper allowed the consultants to arrive in the capital refreshed and ready for their morning presentations. ### 3. unexpected success noun · CEFR B2 a book, movie, or product that becomes a big success after being ignored for a long time. For teachers: a commercial or artistic work that achieves significant success only after a period of relative obscurity. - The low-budget horror movie was the sleeper hit of the summer. - Industry analysts were stunned when the indie title became a sleeper success, eventually outselling the year's most anticipated blockbusters. ### 4. undercover agent noun · CEFR B2 a spy who lives a normal life in a country and waits for a long time before starting their secret work. For teachers: an agent or spy placed in a target country or organization to remain dormant until activated for a specific mission. - The police discovered that the quiet neighbor was actually a sleeper for a foreign agency. - Intelligence officials suspected that several sleepers had been embedded within the tech firm years before the data breach occurred. ### 5. railway tie noun · CEFR B2 · British English the heavy pieces of wood or concrete that support the metal tracks of a railway. For teachers: a rectangular support for the rails in railroad tracks, typically made of treated wood, concrete, or steel. - Workers were busy replacing the old wooden sleepers with new concrete ones. - The derailment was attributed to the degradation of the wooden sleepers, which had rotted after decades of exposure to the elements. ### 6. modified car noun · CEFR C1 · Slang a car that looks like a normal, slow car but has a very powerful engine inside. For teachers: a vehicle that has been high-performance modified but retains an unassuming, stock exterior to deceive others about its speed. - His old family sedan is a total sleeper; it can beat most sports cars in a race. - He took pride in his sleeper, enjoying the look of shock on other drivers' faces when his rusted hatchback left them behind at the light. ### 7. clothing noun · CEFR B2 · American English a one-piece suit for a baby or child to wear while sleeping, often covering the feet. For teachers: a one-piece set of pajamas for an infant or young child, usually featuring enclosed feet. - The baby looked adorable in his blue fleece sleeper. - She bought a set of cotton sleepers for the newborn, ensuring they had snap closures for easier diaper changes at night. ### 8. sedative pill noun · CEFR C1 · Slang · Informal a pill that helps you fall asleep. For teachers: a pharmacological agent, such as a sedative or hypnotic, administered to induce sleep. - He had to take a sleeper to deal with his jet lag after the long flight. - The doctor warned against the long-term use of sleepers, citing the risk of dependency and altered sleep cycles. ## Examples - The low-budget movie became a sleeper hit after several months. - Critics ignored the novel when it first came out, but it proved to be a sleeper and topped the charts a year later. - While the blockbuster failed to meet expectations, the independent documentary emerged as a sleeper, gaining momentum through word-of-mouth recommendations and eventually securing a national theatrical release. --- From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.