It's when you have a late night conversation (usually while on the phone), while very sleepy. The next morning, you forget what you were talking about towards the end of the conversation.
When you intend to take a short nap before going to your hawt girlfriends house to hang out with her and her cats after work, but you forget to set an alarm and end up waking up around 10pm. Then you just go back to bed until the morning