The connection between sane living and sound work is a physiological and psychological necessity.

Obligate nasal breathing describes a physiological necessity to breathe through the nose (or other forms of external nares, depending on the species) as opposed to the mouth.

Row upon row of quizzical faces tilt to one side, as if this were a physiological necessity when processing strange data from a most unlikely source.

That is when the gravid female, myself, because of the physiological necessity for avoiding further sexual contact with my life-mate, changes gradually into male mode and, concurrently, my mate slowly becomes female.

He had mastered the texts explaining the physiological necessity for sleep among organic life forms, but certain psychological aspects still puzzled him.

Because of the physiological necessity for avoiding further sexual contact with its life-mate, a gravid Hudlar female changes gradually into male mode and, concurrently, its life-mate slowly becomes female.

He had been under massive sedation for almost a month until it became a physiological necessity to taper him off; and as soon as he was alert enough to do so he contrived to hang himself in his room.

Suppose further that he acknowledged sleep a physiological necessity.

Gravity, real or artificial, is a physiological necessity for this life-form.