Eight hours at irregular times is not eight hours.

Sleep research has spent decades telling you to get 7 to 9 hours. Guidelines, apps, wearables, all pointing at duration. A systematic review synthesising 59 studies across diverse populations, makes a strong case that this framing is incomplete.
https://pubmed.ncbi.nlm.nih.gov/41259946/

The breadth of associations is striking. Irregular sleepers show higher rates of depression and anxiety, elevated BMI, insulin resistance, hypertension, and incident cardiovascular events. Prospective biobank data in over 82,000 adults linked low sleep regularity to a 26 to 53% increase in dementia risk and measurably smaller hippocampal volume. In a multi-ethnic cohort, sleep timing variability above 60 minutes predicted a doubling of incident cardiovascular events. Five independent cohorts showed 20 to 88% higher all-cause mortality in the least regular sleepers, independent of both duration and quality. These are associations, not proven causal chains. But they are consistent, across continents, age groups, and measurement methods, and that consistency points somewhere.

The most credible causal culprit is circadian misalignment. When your sleep timing drifts day to day, your internal biological clock loses its anchor. Think of it as your body expecting the night shift to start at the same time every evening so it can initiate repair, hormone release, immune activity, and metabolic cleanup in the right order. Irregular timing means those processes start late, overlap, or get skipped entirely. The downstream signal is measurable: higher circulating IL-6 and TNF-alpha, disrupted glucose regulation, elevated evening blood pressure, and suppressed parasympathetic tone.

If you have written off sleep anxiety as a soft concern, this review names it formally. Orthosomnia, the compulsive pursuit of perfect sleep metrics, is now a recognised clinical phenomenon. And here is the sharper point: if you are losing sleep over hitting exactly 8 hours, you are optimising the wrong variable entirely.

Regularity gives you more return of investment and is actually much easier to achieve. A consistent sleep window, within roughly one hour each day, stabilises the circadian system more effectively than any number of hours logged at the wrong time.

Duration is what you sleep. Regularity is when.
Be mindful of the latter.