Your dog spends about 20% of the night awake and its sleep quality doesn't drop at all. In the bed-sharing arrangement, you were always the fragile one. Dogs are polyphasic sleepers. Their sleep comes in short bouts stacked across the whole day, and their brains are built to surface, check the room, and sink back under in seconds. Humans run on 90 minute cycles where a single bad 3am wake-up can cost an hour. Same mattress, two completely different operating systems. A Mayo Clinic study put activity trackers on 40 owner-dog pairs for a week. The dogs hit 85% sleep efficiency whether they slept on the bed or the floor. The humans managed 81%, and their number moved depending on where the dog was. Dog sleep was bulletproof. Human sleep bent around the dog. Then wearables blew up the sample sizes. A 2026 analysis of 11,733 WHOOP users across 1.6 million nights found that on dog-in-the-room nights, people slept about 10 minutes longer, resting heart rate dropped about 1 beat per minute, and heart rate variability rose. There were a few extra micro-wakeups, around 0.4 per night. The nervous system downshifted anyway. It read the dog as safety. A 2025 study ran sleep EEG on family dogs and found the inversion. Dogs sleep measurably worse when their owners aren't in the room. You are your dog's sleep aid, and apparently its main sleep disruption too. The evolutionary read is that this deal is tens of thousands of years old. Humans who slept near dogs could afford deeper, more vulnerable sleep because something with better hearing was handling the night watch. Co-sleeping researchers think dogs literally expanded how much uninterrupted sleep early humans could get. The dog takes the broken sleep so you don't have to. The trade predates agriculture, and your heart rate still drops a beat the moment one curls up at your feet.