Waking groggy after eight hours of sleep may be due to sleep cycles

A sleep expert, Dr. Lindsay Browning, says waking in the middle of a roughly 90‑minute sleep cycle can cause sleep inertia, and timing sleep to about 7.5 or 9 hours is more likely to coincide with lighter sleep stages.

I was sleeping eight hours each night but still waking exhausted and foggy. Dr. Lindsay Browning, a neuroscientist and sleep expert, explained that sleep is organized into cycles made up of light sleep, deep sleep and REM. Each cycle lasts about 90 minutes on average. Waking during a deep part of a cycle is linked to sleep inertia, the groggy feeling after awakening. Key points: - Sleep is structured in cycles of light, deep and REM stages that repeat roughly every 90 minutes. - Being woken from deep sleep is associated with sleep inertia, which can cause pronounced grogginess. - Dr. Browning explained that an alarm that lands in the middle of a cycle is more likely to produce groggy wakeups. - The article reports the expert suggested aiming for wake times around 7.5 or 9 hours to better align with cycle endpoints. - The expert also discussed sunrise alarm clocks, morning daylight exposure and a consistent sleep schedule as ways to support waking and circadian timing; the author noted improved wakefulness after using a sunrise alarm. Summary: Aligning wake time with the end of a sleep cycle may reduce sleep inertia and lessen morning grogginess. The expert mentioned sunrise alarms, morning light exposure and consistent sleep timing as supportive measures to help the body align with natural sleep stages.