On Earth people's lives are usually dictated by a 24-hour rhythm (with about 8 hours of sleep each night). As far as I know this rhythm is determined by chemical processes in the human body which are influenced by the sun (see [Circadian rhythm](https://en.wikipedia.org/wiki/Circadian_rhythm)).

However dividing time into 24-hour periods isn't natural for astronauts on ISS (which make a turn around the earth in about 90 minutes, usually but not always with accompanying sunrises and sunsets).

Do astronauts aboard the ISS still divide time into 24-hour periods or do they use some other regime? What kind of sleep needs does the human body have in space - e.g. do astronauts still sleep for 8 hours at a time or do they sleep for less/more time more/less often? Are all astronauts sleeping at the same time?