Analysis of 74,733 middle-aged and older adults across six US and European cohorts reveals that consistent sleep-wake patterns significantly protect against dementia. Each standard deviation increase in sleep regularity corresponded to 9-14% lower dementia risk over 7-8 years of follow-up, with 1,906 dementia cases documented. Three distinct measures of circadian consistency — rest-wake regularity, sleep-wake timing, and overall 24-hour activity patterns — all demonstrated protective effects. This finding represents a major advance in understanding how circadian rhythm stability influences brain health. Unlike previous inconsistent research, this study's massive scale and international scope provides compelling evidence that sleep regularity may be as important as sleep duration for cognitive preservation. The protective effect appeared strongest in adults under 65, suggesting early intervention potential. However, the observational design cannot establish causation, and individual variations in optimal regularity remain unclear. As a preprint awaiting peer review, these results require validation, but they strongly suggest that maintaining consistent sleep schedules could be a practical, accessible strategy for dementia prevention — potentially more achievable than complex interventions targeting sleep quality or duration.