This was originally posted on All Things Crabby: http://www.allthingscrabby.com/blog/index.php/2009/10/25/do-my-crabs-need-a-light?blog=3
Yes they do.
Light and proper food fuels a hermit crabs metabolism. A hermit crab’s metabolism enables them to adjustment to their surrounding environment. Hermit crabs require a normal cycle of day and night to maintain their metabolic functions. Along with a nutritious, balanced diet. A cycle of 12 hours light and 12 hours night is commonly recommended and used. With overhead lights you will notice that your crabs will be more active during the day. We’ve always been told that because crabs are nocturnal, they are not active during the day. Try turning a light on them and you will see how false that is. Hermit crabs are primarily nocturnal but not exclusively.
Your overhead light can also be your heat source but this can be it more challenging to maintain your humidity. I recommend you use a set up with a day bulb and night bulb. You can purchase a bi light hood that will accommodate two bulbs that are controlled with two switches. This means you can buy a couple timers and set your lights on a timed cycle to come off and on as needed. The wattage of the bulbs will be determine by the size of your tank and if you are using the bulbs for your heat source. If you don’t need the bulbs for warmth, go with a very low wattage. You are looking for bulbs that provide UVB rays. These are bulbs sold for reptiles.
Even molting crabs should be kept on a normal cycle. This flies directly in the face of advice we have been given for several years. Molting is a metabolic process. Now if you consider molting in the wild, a crab simply digs down and molts. It doesn’t go into a dark cave or den, then dig down and molt. When the crab digs down, it is obviously in the dark but there is no reason to deny the crab of light pre or post molt.
Sue Latell of Coenobita Research has been conducting a light study and continues to research the benefits and affects of light cycles on hermit crabs. This information has been derived from her current research articles or conveyed to me directly during our discussions on hermit crab care.
The relationship between light, food and a hermit crab’s metabolism and how it plays a role in environmental adaptation, is exclusively the result of Sue Latell’s many hours of research and study. Sue’s ability to decipher and apply research papers and her consultations with experts in the field, have revealed some large flaws in our previous methods of crab care. It is always an effort to change the popular, common methods but in the case of hermit crab care, it is vitally necessary to improve the quality of life we provide to our hermit crabs.
In lab rearing conditions a constant light cycle of 14 hours of light and 10 hours of dark was maintained.  You can easily control your day/light cycle with a timer power strip.
1. Abbreviated larval development in the Australian Terrestrial Hermit Crab Coenobita variabilis by Alan W. Harvey