lighting should simulate the sunlight they get in their natural range.
Natural sunlight does lots of things for most animals- it lets them see
clearly; it helps develop their eyes; it helps program their brain for
daily and seasonal biorhythms; it helps regulate breeding and other
behaviors; it helps control the growth of germs and such; it keeps them
warm; and more.

Tortoises raised in improper lighting usually
do not show any clear evidence of problems but may not grow or
reproduce as well as a healthier tortoise. One on-going debate, for
example, is the importance of UV, especially the range known as ‘UVB’

Natural sunlight is made up of several elements, many that we cannot see. The important elements for us include…

  • Infrared light (IR), a very long wavelength (10,000 to 700 nanometers) ‘below’ the color red, is mostly used for heat. IR
    offers a benefit by warming the item it shines on more than it does the
    air in-between. It also penetrates more ‘deeply’ than many other
  • Visible light- red, orange, yellow, green, blue, and violet- is a range of
    wavelengths from the longest to the shortest (700-400nm). The human eye
    has three structures that see color and ‘trigger’ at reddish, greenish,
    and bluish. This means that if you shine the right combination of red,
    blue, and green, it will look pretty white to a human.

     The problem is that it is not ‘really’ white. Light that looks right to us
    does not always look right to cameras, plants, and other things. It
    does not even really look right to us if we compare it to a true white.
        The problem is a bit worse for tortoises. they have four
    structures in their eyes to see color and are thought to be able to see
    some ultra-violet, like insects and other animals can. What looks white
    to us under standard fluorescent lighting probably looks grey-red to them. This can affect behaviors, food recognition, and more.
  • Ultra-violet light (UV) is a shorter wavelength (400-100nm) past purple. There is actually a broad range of UV light-
    • UVA
      is the longest wavelength of the group (400-315nm) and is closest to
      purple. It is known to have effects on animal behaviors and on the
      pineal gland and other organs- even in humans.
    • UVB is the
      middle range of wavelengths (315-280nm). This is the band that is
      needed by most animals to make vitamin D in their skin. (Well, not
      really, but the real process is pretty complicated. You can read it here.)
      It generally does not take a lot- it makes about 1,000 IUs of vitamin D
      a minute with good sunlight, so it only takes a few minutes every
      couple days to get a good dose.
           One problem is that the range of UV that does this is close to 280nm, or almost…
    • UVC,
      the shortest and most harmful wavelength (280-100nm). Most of this is
      filtered by the atmosphere, which is good news. UVC is great at killing
      things- cells, germs, etc.

UV light is a mixed blessing.
It helps promote behaviors, makes vitamin D, kills germs, and more. On
the other hand, it causes sunburns, skin cancer, eye damage,
degradation to plastic and cloth, and so on.

sunlight is by far the best lighting option for tortoises. If a
tortoise can be offered about an hour’s worth of good sunlight (light
that does not pass through glass, plastic, or fine screens and is
strong enough to tan a person) in a week, over several short sessions,
then they only need simple lighting in the habitat.

How much light?
This depends on several things.

  • Age/size. Young tortoises spend most of their time hiding from light and predators so would need less light than older tortoises would.
  • Natural range- latitude.
    Tortoises near the equator get more direct, stronger light than
    tortoises from outside the tropical zone. Once you get past 40 degrees
    (about Boston in the US), the sunlight is so weak that little UVB
    reaches us in the winter.
         Latitude also affects
    day length. At the equator, days are generally about 14 hours long,
    while further away the days get shorter, and there is more seasonal
  • Natural range- climate and overhead vegetation.
    The more overcast or overgrown a place is, the weaker the light hitting
    the ground. Open desert gets plenty of unfiltered light, while a deep forest would only get about 10% on the ground.

matter the age or species, all tortoises spend much of their day
hiding, so there should always be plenty of shade, hides, and resting

baseline wattage can be figured out by multiplying the
length and width of the space, in feet, then multiplying that answer by
1.5. My 48in x 20in ‘tortarium’ habitat is about 8 square feet.
Multiply that by 1.5 and I need about 12 watts to light it to a
comfortable level. This seems rather low, so the formula may not work
for smaller spaces so you might double or even triple it for smaller

The best options
There are three main general purpose indoor lighting plans we can use to best simulate natural sun at reasonable costs.

  • Florescent bulb option.
    Can be used for small to medium habitats. May require a separate heat
    element. Studies with Panther Chameleons showed this was the best set
    up for reproductive success in that species, but the conclusions seem appropriate here as well.
    • Start
      with a low-level UVB-style fluorescent bulb sized appropriately for the habitat
      and hung per directions. Set to run for about 6-8 hours in the middle of
      the day.
    • Supplement
      with a warm white, ‘true white’, deluxe white, or ‘daylight’ fluorescent
      bulb or a standard incandescent bulb set to run for 12 hours, or 10 hours in the winter and 14 hours
      in the summer.
  • “Four tube” option. Four old-style, thicker, 48″ or longer T-8 bulbs emit
    enough UVB light that no other lighting may be needed. The bulbs should be
    daylight, deluxe white, sunlight  or superwhite, although a mix of
    plain warm and cool white will also work.
    • If possible, the lights should come on and go off over a period of time to better simulate dawn and dusk.
  • Mercury vapor option.  An inexpensive and simple option for a medium-sized habitat. It can create a lot of heat and light in a smaller habitat. The intensity created may be an issue for any small tortoises or forest tortoises.
    • Start with a Mercury Vapor bulb with UVB capabilities.
      • Wattage appropriate to the habitat size.
      • Hang at least 12in above the tortoise backs- higher if needed to regulate temperatures. 
      • Set to run for about 6 hours in the middle of the day period.
    • Add
      a second bulb to offer more color balance, as well as a dawn/dusk
      simulation. This can be a plant bulb, a warm white (or true white, deluxe white or daylight) fluorescent, or a
      standard incandescent.
      • Wattage appropriate to the habitat size.
      • Height over the habitat is relatively unimportant.
      •  Set to run for 12 hours, or for 10 hours in the winter and 14 hours in the summer. 

of the options is improved by exposure to sunlight, even through a
window, to help offer balance, dawn and dusk, etc. (Although remember- useful UVB does penetrate glass!) Make sure that the
habitat does not overheat because of the sunlight.

Measuring light quality
We often measure
light in ‘Color Temperature’ and ‘Color Rendering Index’ (CRI). While
the official descriptions are rather complicated, the simple versions

  • Color Temperature
    The hotter a source, the more it will appear bright blue. Our sun, as
    seen from Earth’s surface, ranges from 5,000 to 6,500 Kelvin, depending
    on distance and atmospheric conditions. A typical  incandescent
    bulb runs about 3,000K- about twice that of a candle. In general, the
    higher the color temp, the more ‘sun-like’ it will appear.  
  • CRI– The higher the CRI, the more truly white a
    light is. If you shine a high CRI light through a prism, or bounce it
    off a CD surface, you get a nice, blended, even rainbow. The lower the
    CRI is, the more incomplete of a rainbow you get.It is surprising how low most of our common lights are on the CRI.
  • Note-
    many UVB bulb makers also list some sort of rating for the UV output.
    These ratings are mostly useless except for comparing one of their
    bulbs against another- like a UVB-10 would probably offer twice as much
    UVB as a UVB-5, but that is not really a scientific standard.

Something to think about…

  1. ALL light a wild animal receives contains IR
    (heat), visible light, UVA and UVB. Breaking it apart into different
    bulbs as we sometimes do may interfere with natural instincts and
    development. This is a reason to consider using low-level UVB for all lighting in a reptile habitat.
  2. Tortoises
    can see colors and UV light, so can see red, blue, and black
    ‘night-time heat bulbs’, which may interfere with natural sleep