Final answer:
The phrase 'to sleep in' means to oversleep, as opposed to other related terms like 'to lay down' or 'to lie down.'
Step-by-step explanation:
The phrase to sleep in means to oversleep or to sleep later than usual, especially past the time one usually wakes up or past the time of one's obligations like school or work. This commonly occurs when one incurs a sleep debt. It's important to distinguish this phrase from other sleep-related terms. For example, 'to lay down' means 'to put or set something down,' while 'to lie down' means 'to be in or move into a horizontal position' or 'to be situated,' with 'lay' also being the past tense of 'lie.' Understanding these phrases is helpful for grasping the nuances in English related to rest and sleep patterns, as well as the importance of sleep on physical and mental health.