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Do Tortoises Need Direct Sunlight?

Last Updated on October 9, 2023 by Samantha Harris

As cold-blooded reptiles, tortoises rely on external heat to regulate their internal temperature.

This usually comes from exposure to direct sunlight. Tortoises rely on UVB rays, basking for many hours as part of their routine. If you keep a tortoise indoors, this won’t happen.

A tortoise without heat won’t be able to regulate its bodily functions, leading to digestive, shell, and organ problems. Also, if a tortoise doesn’t receive UVB rays, it’ll be unable to synthesize vitamin D3, have trouble sleeping, and its metabolic processes will cease.

Direct sunlight is one way to resolve the problem, but it’s not the only option.

You can get UVB heat lamps that replicate the effects of sunlight indoors, like mercury bulbs. This can be safer than sunlight because it won’t risk overexposure, leading to heatstroke.

Why Do Tortoises Need Sunlight?

No artificial means is better than sunbathing for a tortoise. The American Journal of Veterinary Research shows that tortoises produce more vitamin D3 when exposed to direct sunlight.

Sunlight contains UVB light wavelengths, which trigger the production of Vitamin D3 in tortoises, which plays a vital role in calcium absorption. You can use mercury bulbs, which emit higher levels of UVB.

Calcium is necessary for healthy bone and skeletal formation. Consequently, tortoises deprived of sunlight are likely to experience a calcium deficiency, which can trigger Metabolic Bone Disease (MBD), an umbrella term for various illnesses associated with calcium deficiency.

According to the Journal of Zoo and Wildlife Medicine, MBD is the most common health problem affecting tortoises raised in captivity. Symptoms of MBD include:

Issues like shell pyramiding and bowed legs can’t be reversed. Cloacal prolapse and respiratory infections can be life-threatening to tortoises in the short and long term.

why do tortoises need UV light?

Regulates The Circadian Rhythm

Aside from vitamin D and calcium, sunlight affects a tortoise’s internal body clock.

Also known as the circadian rhythm, this controls a tortoise’s behavior. It’s influenced by sunlight. Tortoises are most active in the day when there’s sunlight and sleep when it’s dark.

Although UVB lamps can recreate this for your tortoise, natural sunlight does it automatically. Placing your tortoise outside in a safe enclosure involves following the natural pattern of nature.

The only risk is if you live in a region that doesn’t provide enough sunlight. For example, many species hail from the equator, which usually experiences 12 hours of light and 12 hours of darkness.

If your day-night cycle has more darkness, supplementing it with a heat lamp is essential.


Since tortoises are cold-blooded animals, they require external heat sources to raise their body temperatures. So, allow your tortoise to bask outside in the daytime for a few hours.

A heat lamp can regulate this, but proper sunlight offers a more natural alternative.

Give a tortoise room to burrow, find shade, and soak in water. Then, it’ll soak up the sun’s rays before heading somewhere cooler.

Is Excess Direct Sunlight Bad for A Tortoise?

However, direct sunlight has its disadvantages. UVB light is essential for healthy tortoises, but you can easily overexpose your pet tortoise.

According to the Journal of Herpetology, tortoises can experience heat stress when exposed to sunlight for prolonged periods. Likewise, over-exposure may cause severe damage to a tortoise’s skin.

Wild tortoises can escape underground or find shade nearby. Your tortoise enclosure may lack this outside, making it dangerous.

Indoor tanks near open windows can take in too much sunlight, creating a pocket of hot air that tortoises can’t escape, even if they move away from direct sunlight.

A heat lamp can be set at a precise temperature, and you can monitor the warmth of a tortoise table using gauges and thermometers.

Direct sunlight is far harder to regulate, as your tortoise’s air, ground, and internal temperatures differ wildly. One oversight can lead to your tortoise getting heatstroke.

Likewise, placing your tortoise near a closed window won’t help. Window glass is often designed to block UV radiation, so your tortoise will enjoy the light but have no advantages metabolically.

If you rely on direct sunlight, ensure it has a well-ventilated enclosure, access to burrowing sights, fresh water, and shade.

You should check on the tortoise once every hour if the temperature is high. Even if it feels okay to you, a tortoise left in the sun with no reprieve can get sick quickly.  

Can A Tortoise Live Without Sunlight?

A tortoise may survive for several months in warm indoor temperatures without a heat lamp. However, it’ll likely have soft shell and metabolic bone disease because it still needs UVB rays to thrive.

It’ll be okay to rely solely on a heat lamp and never give your tortoise sunlight. As long as the bulb provides UVB rays, this will supplement your tortoise’s need for vitamin D and calcium.

Bulbs can artificially regulate its internal clock and will ensure it basks properly. Many tortoises that live indoors, especially in regions with little natural sunlight, experience long and happy lives with artificial UVB rays.

Direct sunlight is a natural and more efficient method. If you wish to boost your tortoise’s mood, bone strength, and energy levels, offer supervised sunbathing whenever possible.

why do tortoises bask?

How Much Sunlight Do Tortoises Need?

Tortoises require 8-12 hours of sunlight each day to remain healthy. Even with a heat lamp, providing less than this can lead to certain tortoise species entering brumation.

However, providing more light than 12 hours of sunlight can cause sleeping issues. It’ll be confused by the extra-long daylight hours, and its internal clock will be disrupted.

Worse still, your tortoise might absorb too much Vitamin D, leading to excess body calcium (hypercalcemia). Also, vitamin D toxicity is associated with renal problems in tortoises.

Alternatives to Natural Sunlight For Tortoises

With the pros and cons in mind, you may choose to forgo sunlight or use it in tandem with artificial lights. This is sensible because UV-emitting bulbs rarely cause skin problems or heat stress.

However, they don’t have the same light gradient as natural sunlight. It takes longer for UVB lamps to provide your tortoise with the same amount of benefits as natural sunlight.

When choosing artificial lamps, consider standard UVB and colored bulbs. Blue, white, and fluorescent lights are brighter and better mimic the lighting conditions tortoises experience in the wild.

However, UVB bulbs can be intense. By contrast, red lights can be used at night to give tortoises extra warmth without interfering with their circadian rhythm.

Place the light bulbs at a distance from the tortoise and only on one side of the enclosure. This will ensure the tortoise can bask and retreat as it needs to.

Don’t place mesh, glass, or plastic between your tortoise and the bulb because this interferes with UVB ray absorption. Pairing this with occasional sunlight as a natural boost can strike the perfect balance.