A behavioral sleep-hygiene principle: the bed should be associated, through consistent use, only with sleep and sex — not work, scrolling, wind-down activities, or lying awake with racing thoughts. Doing other activities in bed weakens the learned mental association between 'being in bed' and 'sleep,' making it harder to fall asleep quickly and reliably.
This complements but is distinct from The Cave Principle: Sleep Environment Optimization (the physical qualities of the sleep environment — dark, quiet, cool) and Wind-Down Routine and Parasympathetic Activation (pre-sleep activities to calm the nervous system): stimulus control is about where activities happen, not the environment's physical properties or an activity's physiological effect.
Dr. Cheri Mah (NBA/NFL sleep consultant) frames the rule sharply: "sleep and sex are all that you want in bed everything else should be outside." Work, screens, journaling, and rumination should happen in another room so the bed keeps its single behavioral association with sleep onset. See Wind-Down Routine and Parasympathetic Activation for where the pre-bed processing should happen instead.