That is when you fly the last flight in and first flight out in the morning, with maybe 5-7 hours in a hotel because you are technically on duty all night and off all day when you get back to base. If your evening flight is delayed it can be an even shorter night. Some people who live in base like them because they are home more than four day trips and it’s fairly easy work for minimum guarantee. Those who don’t nap well during the day hate standup overnights.