Horses are able to rest while standing, yet rapid eye movement sleep requires complete muscle relaxation in recumbency. This biological necessity reflects the species’ evolutionary balance between vigilance and recovery. When horses do not perceive their environment as comfortable and safe to lie down, chronic sleep restriction may develop and, in severe cases, result in atonic collapse. While individual cases have been described, large-scale developments within stable social groups have not previously been reported. We monitored 22 mares in 4 groups after relocation from an unstructured group housing to a paddock-trail group housing system that exceeded Swiss legal welfare standards. We recorded lying behaviour with limb mounted sensors continuously over 23 months, yielding 4179 days of data. Mean daily recumbency declined from 29 ± 55 minutes at initial monitoring at the new site to 15 ± 30 minutes in the final phase. After two years, only 22% of mares achieved at least 20 minutes of recumbency per day. Targeted management modifications, such as an alteration of feeding management towards ad libitum feeding, access to pasture or shelter modification (open-sided shelters, extended sand areas, or both), did not increase lying duration. In a different housing environment (partially single housing, partially unstructured group housing) however, the same horses averaged 126 ± 81 minutes per day in recumbency. During the prolonged period of reduced lying, several mares developed clinical signs related to atonic collapse. These findings document a sustained, group-wide reduction in recumbency following relocation, despite structural compliance with high welfare standards. The magnitude of apparent sleep deprivation observed across an entire group has not previously been described. The results demonstrate that regulatory standards alone do not guarantee fulfilment of the natural sleep requirements of horses. Objective monitoring of recumbency should therefore be integrated into welfare assessment of group housing systems. Lay Message Horses must lie down to achieve “dream sleep” (REM-sleep). After moving to a new housing system, many mares almost stopped lying down, and many showed signs of collapses related to sleep deprivation. This occurred even though the stable exceeded legal welfare standards. The results show that good infrastructure is not sufficient if horses do not feel comfortable or secure enough to lie down. Monitoring lying behaviour can help identify serious welfare risks early.