Whos been sleeping in my bedroom? Sleep and cardiorespiratory physiology when co-sleeping with a dog, cat, or child

Co-sleeping with pets or children is common, yet its effects on sleep, cardiorespiratory physiology, and behavioral outcomes are understudied. We examined within-person associations between co-sleeping with a dog, cat, or child and sleep characteristics, measures of cardiorespiratory physiology, and next-day physical activity in 1,649,083 person-days from 11,733 adults wearing the WHOOP wearable device. Participants reported nightly co-sleeping via a daily journal in the devices companion smartphone application, and linear mixed-effects models compared nights with and without co-sleeping within the same individual. Co-sleeping was associated with modest improvements in cardiorespiratory physiology, including lower resting heart rate (0.8-1.1 beats/min), lower respiratory rate (0.04-0.07 breaths/min), and higher heart rate variability (1.41-1.95 ms). Sleep outcomes were mixed, with longer sleep duration (5.5-9.6 min) but more disturbances (0.36-0.40 instances) and slightly less restorative sleep (0.23-1.17%). Associations were generally consistent across groups, although child co-sleeping showed greater sleep disruption. Next-day physical activity was higher following dog and cat co-sleeping (7.6 and 7.4 intensity-weighted min, respectively) but lower following child co-sleeping (2.6 intensity-weighted min). Although effect sizes were small ({beta} range: 0.008-0.045), findings suggest that co-sleeping is associated with a trade-off between modest cardiorespiratory benefits and mild sleep disruption, indicating that co-sleeping decisions may be driven more by personal and contextual factors than by concerns about physiologic impact.