Recommended sleep environment is summarized as a "cave": dark, quiet, cool, and comfortable — four independently controllable variables, each with a specific fix.
Presented as a checklist of controllable, largely one-time environmental fixes, contrasted with behavioral protocols like Pre-Sleep Shower/Bath Timing and Core Body Temperature and Wind-Down Routine and Parasympathetic Activation that have to be repeated nightly.
Concretely, "cave" conditions map to specific tools: dark — blackout curtains or an eye mask; quiet — earplugs or a white noise machine; cool — a bedroom held at 60–67°F; comfortable — a mattress/bedding setup suited to the sleeper, including a mattress cover so partners with different temperature preferences (one runs hot, one runs cold) can each get their own side right.
To find your own optimal point inside the 60–67°F band, use a temperature-finding protocol: lower the room by 1–2 degrees every couple of days and track how it affects your ability to fall and stay asleep, rather than guessing at a single target number.