Replace a compact UVB bulb every six months and a linear tube every twelve months. Ultraviolet output fades steadily through a bulb's working life, and it drops below a useful level long before the visible light looks dim or the bulb fails. A reptile can therefore sit under a bright, apparently healthy light while receiving too little ultraviolet to process calcium properly, and the shortfall builds quietly over months rather than announcing itself in days.
Why the Glow Misleads
Your eyes judge a bulb by its visible brightness, and that is the wrong measure for this job. The phosphors and gases that generate ultraviolet light degrade faster than the parts that produce visible light, so the two outputs follow different timelines inside the same bulb. A tube that still lights the enclosure evenly may deliver only a fraction of its original ultraviolet at the basking spot. Distance matters as well, because ultraviolet weakens quickly as the gap between bulb and basking surface grows, and a mesh lid between the bulb and the animal filters out part of what remains. Replacing on a calendar interval, rather than on appearance, removes all of that guesswork in one move.
Intervals by Bulb Type
Use six months for compact bulbs, the coiled or U-shaped types that screw or plug into a dome or canopy fitting. Use twelve months for linear tubes, the straight fluorescent types that sit in a reflector fixture along the enclosure. Fit the bulb at the distance the maker specifies for your species, and measure that distance from the bulb face to the top of the basking surface rather than to the floor. If the maker's stated life for your exact bulb is shorter than six months for a compact or twelve months for a tube, follow the shorter figure, because the maker's rating for that model outranks a general interval. Note the fitting date before the bulb goes in, while the base is still cool and clean enough to write on.
A Tracking System That Holds
Three steps, done together, make the interval self-managing. First, write the fitting date on the bulb base or fitting in permanent marker on the day the bulb goes in. Second, log that same fitting date in the animal's own records, alongside feeding, shedding, and weight notes. Third, set a reminder for one month before the due date, which leaves time to buy the correct replacement and fit it without a gap in ultraviolet cover. When the new bulb goes in, repeat all three steps for that bulb rather than editing the old entry, so the record shows an unbroken chain of fitting dates. Keep the old bulb out of the enclosure once it is swapped, since a retired bulb put back by mistake restarts the problem silently.
Signs the Schedule Has Slipped
Treat a missed interval as a husbandry fault to correct promptly, not as an emergency you can diagnose at home. An animal kept too long under faded ultraviolet may show reduced appetite, lower activity, or a reluctance to bask, and each of those signs has other possible causes as well. Restore a fresh bulb at the correct distance, check that basking and cool-end temperatures are where they should be, and review feeding and supplement routines against current guidance for the species. If appetite or activity does not recover, or if you notice tremors, soft or misshapen limbs, or difficulty moving, book a reptile veterinarian promptly, because those are signs a vet needs to investigate in person. KinsLe's Care Teams supports that follow-through by letting you assign the monthly bulb-date check to whoever shares care of the reptile, so the fitting date gets read and acted on even in a busy week.