Sleep Drift
Bedtime has probably moved. Ten o’clock becomes ten-thirty. Ten-thirty becomes eleven. The change is gradual enough that you barely register it.
A second wind at night feels productive so you answer messages, you finish tasks and you consume content and before you realise it, midnight lights out becomes ordinary.
The sleep that follows is often more fragmented and you wake earlier than you intend. You often wake during the night with a sense of alertness that does not match the hour and even if you sleep through, you wake feeling as though your system never fully powered down. In your late thirties and forties, hormonal rhythms that once stabilised sleep begin to shift alongside circadian timing.
Circadian rhythm is shaped by light exposure, stimulation and timing. Cortisol should rise within the first hour of waking to create clarity and forward drive to get you out of bed and melatonin should rise in the evening as light reduces making you sleepy for bedtime. When your bedtime shifts later and artificial light extends into the night, that rhythm adjusts.
Cortisol follows a predictable curve across a 24-hour period. One of its defining features is the Cortisol Awakening Response, a sharp rise within the first 30 to 60 minutes after waking that generates clarity and momentum. When sleep becomes compressed or fragmented, that morning rise can flatten or delay. You may still wake on time, but without the same internal ignition, and over time evening alertness begins to replace morning drive.
In earlier years, the system absorbed that reversal. Oestrogen supported resilience and progesterone supported down-regulation, allowing recovery to occur even when rhythm was inconsistent. As ovulation becomes less reliable and progesterone declines, that buffering effect weakens. Compressed sleep no longer disappears into the background but accumulates as sleep debt.
Sleep debt builds gradually through late nights, early alarms and fragmented rest. Over time, that deficit increases stress sensitivity, alters glucose regulation and reduces emotional steadiness, amplifying the contrast you are already noticing. A late night is more likely to register the following day through shortened patience, blurred focus and a narrower emotional range.
In this phase, sleep can no longer be approached with the flexibility it once tolerated, because it underpins cognitive clarity, metabolic regulation and nervous system stability. Protecting it becomes one of the most direct ways to steady rhythm and preserve capacity across the month.
When sleep destabilises, blood sugar stability and stress chemistry tend to follow.
This article is part of the Resilience Series.
Start here: When resilience changes.