Sleep in Definition 1 of 2Pro Tip 1/3Weekends and Holidays Used to express leisurely mornings on days off from work or school. I'm going to sleep in on the holiday. Pro Tip 2/3Not Negative Doesn't necessarily imply laziness; it's often planned and enjoyed. After a long week, I deserve to sleep in. Pro Tip 3/3Casual Use Common in casual, everyday conversation rather than formal use. Let's sleep in tomorrow and relax.