回笼觉 unprotected sleep . Unprotected sleep is going back to sleep after turning off your alarm clock, risking not waking up for a job, class, or other daily task.

你已经起床或者你已经醒来准备起床，但是你因为太累或者不想起床想去睡觉，这次的睡觉就叫做回笼觉。

to fall asleep again after wakening

[steam again]∶把已蒸熟而冷了的馒头、包子等放回笼屉再蒸. so 回笼觉 means you woke up, you want to sleep a little more, so you fall asleep again --- 回笼觉.

It means when you get up for a while but you still feel so tired, then you go back to sleep. The time interval cant be so long.