Catherine E. Milner and Kimberly A. Cote, “Benefits of Napping in Healthy Adults: Impact of Nap Length, Time of Day, Age, and Experience with Napping,” Journal of Sleep Research 18, no. 2 (2009): 272–281; Scott S. Campbell et al., “Effects of a Month-Long Napping Regimen in Older Individuals,” Journal of the American Geriatrics Society 59, no. 2 (2011): 224–232; Junxin Li et al., “Afternoon Napping and Cognition in Chinese Older Adults: Findings from the China Health and Retirement Longitudinal Study Baseline Assessment,” Journal of the American Geriatrics Society 65, no. 2 (2016): 373–380.