In plain terms: Sleep duration & quality (as longevity target) shows early signals in human research but the evidence is limited -- small trials, surrogate endpoints, or not yet independently replicated. It is an emerging area, not yet a proven one.
Sleep duration & quality (as longevity target)
The evidence as the record states it
Zhu et al. (European Journal of Cardiovascular Nursing, 2022; PMID 34279625): 13 reports from 11 sleep extension trials; overall no significant change in blood pressure, fasting glucose or insulin resistance, with lower blood pressure only in a subgroup with raised pressure. Beals et al. (Diabetes Care, 2026; PMID 41564347): 29 short sleepers given about an hour more sleep a night for 6 weeks; no change in clamp-measured insulin sensitivity or glucose control. Hartescu et al. (Journal of Sleep Research, 2022; PMID 34459060): 18 overweight or obese men, 6 weeks; blood pressure and HOMA-IR fell, in a trial too small to change a grade. Gonzales et al. (Sleep Medicine, 2022; PMID 35180478): 12 middle-aged adults who were not short sleepers, five nights of extended time in bed; blood pressure and arterial stiffness unchanged, forearm vascular capacity higher. Tasali et al. (JAMA Internal Medicine, 2022; PMID 35129580): 80 adults with overweight who slept under 6.5 hours; about 1.2 hours more sleep and 270 kcal a day less energy intake, in one unblinded trial. Mixed and unreplicated results on surrogate markers: Grade C. Mortality: no randomized trial has tested sleep extension against death. The link between sleep length and mortality comes from cohorts (Cappuccio et al., Sleep, 2010; PMID 20469800; short sleep RR 1.12, long sleep RR 1.30, sleep self-reported), and observational data never carries a grade.
Evidence by specific use
Why Grade C and not Grade B
Grade C reflects early or limited human evidence -- small trials, surrogate endpoints, or preliminary findings not yet independently replicated. The evidence points in a promising direction but does not yet meet the standard for Grade B. A single well-powered, independently replicated trial with a meaningful endpoint would move this to Grade B or higher.
What would move this grade
A well-powered randomized trial with an objective, clinically meaningful primary endpoint and at least one independent replication would support Grade B or higher.