Cacti on average require about 20% more light than succulents.
Succulents thrive when exposed to between 3000-4000 FC(foot-candles)whereas cacti do best when exposed to between 4000-5000 FC. Foot candles are defined as a unit of illumination equal to that given by a source of one Candela at a distance of 1 foot which is basically a fancy old fashion unit of measurement. The easiest way to measure footcandles is by using a light meter.
Too much direct sunlight can cause a succulent to experience sunburn just like us humans and this happens when a plant is moved from an indoor or shaded area abruptly into unobstructed sunlight. One of the easiest ways to permanently damage a beautiful tender succulent is to leave it in the direct sunlight for too long.
The best way to avoid burning your plant is to keep them in an area that gets partial-sun for the majority of the day or full-sun for several hours a day at most. If your only option is to keep your succulents in full-sun make sure to introduce them to it gradually. Start out by providing full-sun for an hour a day and increasing the exposure time in small increments depending on how they react.
Succulents that do well in full sun include agaves,sedums, aloes and echeverias but they should still be introduced to that level of exposure slowly.
At the opposite end of the spectrum you have low light conditions or under exposure where your plants are just not getting enough sun and in that case you’ll see your plants start to stretch out or as we call it etiolation. Etiolation can happen to plants outside and indoors but is most common in indoor plants. One of the first signs that you’ll see is the growth center will begin to twist and turn trying to find a light source. Secondly, your succulent will begin to grow taller and more stretched out. Going from a nice little compact rosette into a tree-like form.
Another issue when succulents are grown in low light conditions is the increased risk of disease. Sunshine is the best disinfectant. Low light areas have less air circulation. Lack of light and air circulation means the soil stays wet longer and wet soil is the perfect breeding ground for disease.