They knew humans could last a long time without sleep, but most had regular sleep intervals. When choosing a new man, they heard he was very reliable, fiercely protective of whatever crew he served, and anyone who'd worked with him were more than a little disappointed when he would be reassigned. Things seemed to be working out fine, until they noticed he was sleeping a lot less than most humans, but when he did, he crashed hard! So much so it was almost impossible to wake him, short of emergencies. When asked how he was able to be awake for so long, only to collapse like that, he sighed and said one word. "Insomnia." -- DaniAndShali
Compared with the rest of the animal kingdom, Humans just don't stop. -- pre-Shattering thesis on the nature of Human Endurance.
Humans are unstoppable. Many a Havenworlder has marvelled after a three-hour work session that the Ship's Human thinks nothing of doing twice, or even thrice that before going to rest. Some Humans could go for longer in emergencies. Everyone seems to have that one story of how their Human stayed awake for impossible lengths of time. Even their own histories have records of Humans staying awake for frankly phenomenal lengths of time.
Then there's the tale of Human Kat. Even for a Human, he was an odd one. He definitely lead solidity to the rumours abounding about Human Unstoppability. It seemed like he never slept. So much so that his shipmates decided to consult the Human Manual about sleep habits, what was healthy, and what was not. You can definitely imagine their shock and surprise when they discovered that working for forty-eight hours straight and sleeping for twelve was not a healthy sleep cycle. So they held an intervention.