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This is a familiar feeling to me as a sailor; when cruising, every place is new, and at anchor I'm never fully asleep the way I am on land. Any little sound, or the cessation of any regular sound I've gotten used to, and I'm up like a shot with the flashlight trying to identify it. Even the relatively slow rotation of the boat around the anchor as the wind shifts is enough to bring me awake thinking "what changed?"

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It sounds like the brain will only sleep as deeply as it's safe to. That lines up more with the results in the article than the notion that it's directly about how many times you've been in that location before. When you're in charge of a boat, you have to be alert for any danger. When you're a fireman on duty, you have to be ready to hop into your pants. When you're in a strange sleep lab and covered in biosensors, you're going to be creeped out and on-edge.

My grandfather often took the engine room night shift on 'mud barges'. He'd just sleep and any change in sound that might indicate an issue would wake him up instantly. He likes to joke that he got rich sleeping.

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