Cat
The domestic cat's reliability varies according to criteria applied. For feeding-time attendance, cats demonstrate near-perfect punctuality, often arriving several minutes early with vocal reminders. For affection delivery, reliability plummets to negligible percentages. A cat may spend three consecutive evenings upon one's lap, then vanish for a week to pursue alternative arrangements. The cat is reliable in its unreliability—one can depend upon its behaviour being governed by inscrutable internal logic that prioritises feline preference above human scheduling requirements.
Sleep
Sleep operates on circadian rhythms that, under ideal conditions, produce reasonably predictable patterns. The human body generates melatonin according to light exposure cycles, theoretically inducing drowsiness at appropriate intervals. Yet sleep reliability depends upon countless variables: stress levels, caffeine consumption, ambient noise, temperature, and the peculiar tendency of the mind to rehearse embarrassing moments from 2007 precisely when unconsciousness is most needed. Sleep arrives on its own schedule, and that schedule frequently conflicts with human intentions.