A 15 to 20 minute sunbath, 5 times a week, is a reasonable default for an arthritic cat over 12. Two things decide whether it helps or hurts: the UV index at that hour, and the surface temperature of whatever the cat lies on. Through glass, a sunbath is mostly a warmth session. That is still worth doing, because warmth loosens a stiff cat and the ritual gets it moving, but it is not a vitamin D treatment.
Why 15 to 20 minutes
Under 10 minutes most stiff cats have not settled enough to stretch out, so the session ends before the muscles warm. At 15 to 20 minutes the cat is usually loose, has rolled onto a side, and is close to its limit. Past 25 minutes in direct summer sun, a dark fleece bed can pass 40°C at the surface and the cat is now choosing between discomfort and moving, which an arthritic cat does badly. A 20 minute cap also fits the way cats nap: 1 cycle, then up.
The 2 UV numbers
Number 1 is the UV index for that hour, from any weather app. Number 2 is the glazing. Ordinary 4 mm window glass blocks most UVB and passes UVA, so a cat behind glass gets warmth and UVA but very little of the UVB that skin uses to make vitamin D. That is why a window sunbath is a comfort measure rather than a supplement, and why the answer is not a reptile lamp: a 10.0 UVB tube at 30 cm can deliver more UV in 5 minutes than midday summer sun in 30, and thin cat skin on the ear tips burns first.
The 3 joint items
Score 3 items 0 to 3 before the session and again 10 minutes after it. Item 1 is stand-up time: from lying flat to standing, 0 to 2 seconds is normal for a senior cat, 3 to 4 is a change, 5 or more is a flag. Item 2 is steps before the first rest: crossing 6 m without stopping is normal, stopping at 3 m is a change, not crossing 2 m is a flag. Item 3 is grooming reach: can the cat turn and reach the base of the tail, yes or no, checked once a week. A total that improves by 2 points after warmth and falls back by the next morning means the warmth is real but short.
| UV index | Session minutes | Mat surface limit | Best hours |
|---|---|---|---|
| 0 to 2 | 20 | 32°C | Any daylight hour |
| 3 to 5 | 15 | 34°C | Before 11:00 |
| 6 to 7 | 10 | 34°C | Before 10:00 or after 16:00 |
| 8 and above | 0 | Not applicable | Skip, use a warm mat indoors |
| Winter, UV 0 to 1 | 20 | 30°C | 10:00 to 15:00 |
5 sessions a week, at the same hour
Five sessions of 15 to 20 minutes is 75 to 100 minutes a week, and the pattern matters more than the total. Same hour, same mat, same side of the room: cats build the habit in about 10 days, and a habit is what gets a stiff cat off the bed on a cold morning. Skip a day rather than split a session into 2 short ones, because 2 sessions of 8 minutes do not reach the point where the cat stretches out. If the cat asks for the spot on the other 2 days, let it; the 5 is a floor, not a cap.
Set the spot up
A 60 by 90 cm non-slip mat with a 5 cm foam layer, air temperature 18 to 24°C, water within 1 m, and an exit route with no step over 15 cm. Keep the mat 30 cm back from the glass: single glazing in direct sun reaches 45°C on the inside face by early afternoon, and a cat pressed against it is being cooked on one side. No draft between the cat and the glass, and no radiator within 1 m, or the session becomes a 2-source heat load with no way for the cat to step down.
Heat, not sun, is the risk
Stop the session on any of 4 signs: open-mouth breathing, a resting rate over 40 per minute, restlessness that keeps changing position, or the cat moving off the mat on its own before 10 minutes. Flat-faced cats, cats over 6 kg and double-coated cats hit these limits 5 to 10 minutes earlier than a slim short-haired cat. A cat that stays past 25 minutes on a 38°C mat is not enjoying itself, it is stuck.
What to write down
Three columns, one line per session: date, minutes, and the 3 joint scores. Ten lines is 2 weeks. Bring that sheet to the next appointment instead of a description, and compare it with the monthly weight, because 200 g lost over 30 days changes the joint picture more than any mat.
When to book the vet
Book if any joint item sits at 2 or more for 3 sessions running, if the cat stops using the spot it used for weeks, if lameness lasts more than 20 minutes after getting up, or if 4 weeks of sessions produce no change at all. Warmth is not analgesia: a cat that still needs help at 20 minutes after a sunbath needs a prescribed plan, not a longer one.
Do it in 5 steps
- Pick 1 window with 3 or more hours of direct light and check the UV index at that hour.
- Put a 60 by 90 cm non-slip mat 30 cm back from the glass and measure the surface with a thermometer.
- Set a 20 minute timer for the first 10 sessions, then judge by the cat, not the clock.
- Score the 3 joint items before and 10 minutes after, once a week.
- Log 10 sessions over 2 weeks, then keep, move or drop the spot.
Weight is the other half of the joint picture, and the routine is in weighing a senior cat monthly. A cat that sunbathes hard often drinks less, so watch the bowl described in water fountain for a senior cat, and price the annual review in the cost calculator.
Terms mentioned in this article
Frequently asked
Is 20 minutes of sun enough to help an arthritic cat?
It is enough for warmth, which loosens a stiff cat for an hour or two. It is not a vitamin D treatment, because 4 mm window glass blocks most UVB. Judge it by the 3 joint scores before and 10 minutes after, not by how long the cat stays.
Can I use a UV lamp instead of the window?
No. A 10.0 UVB reptile tube at 30 cm can deliver more UV in 5 minutes than midday summer sun in 30, and the thin skin on cat ear tips burns first. Keep the session to window light and a warm mat, and let a vet decide on anything stronger.
What surface temperature is too hot?
Above 34°C on the mat in summer, or 32°C in winter, cut the session short. Single glazing in direct sun reaches 45°C on the inside face by early afternoon, so keep the mat 30 cm back from the glass and check with a thermometer.