Complex sentence: People living by the 28-hour day stay awake for 19 to 20 hours each "day" and sleep 8 to 9 hours; the numbers vary for people with a different amount of sleep needed.
Split into simpler sentences: People living by the 28-hour day stay awake for 19 to 20 hours each "day" and sleep 8 to 9 hours. The numbers vary for people with a different amount of sleep needed.