Three things slip at once, and your night falls out of all three. The patch of cortex behind your forehead thins, and that thinning predicts how much deep sleep you lose ¹. The body clock weakens, so you get sleepy earlier and wake earlier. And the airway loosens: the odds of sleep-disordered breathing roughly triple between 25 and 65 ². Deep sleep itself falls from about a fifth of the night at 25 to under a tenth by 60, with men losing roughly twice as much as women ³.

The architecture is settled; the fixable part is the apnea. Older women whose sleep studies caught apnea had about double the five-year risk of mild cognitive impairment or dementia ⁴. Short sleep tracks with roughly a 28% higher risk of type 2 diabetes ⁵. Most people with apnea never find out they have it.

Screen first, because the biggest lever is the one nobody checks. Four moves, ordered by payback; the pills that most people reach for first come last.

What you get back, and when.

• Weeks: if you treated apnea, the 3pm crash stops being the rule and your partner sleeps again too.
• Months: names come back faster, evening conversation stays sharp, and the creeping A1c and morning blood pressure drift back a notch.
• Years: you keep producing more deep sleep, more consistently, doing the memory-filing job it's for ⁶.

You don't get young-adult sleep back. You get older sleep that isn't quietly bleeding you, which is the larger half of the difference.