Well lit, bright stables are good for the humans who must work in them, and the horses that live in them. Not only should stall, storage and aisle areas be well lit, but all electrical systems and windows that allow natural lighting should be safe.
Safe, well designed electrical lighting is essential; you'll need an electrical panel large enough to run all the systems, and these may include water and heat, that you'll require. This could mean upgrading or adding to your existing system, either by installing a larger panel or sub-panel.
There are many choices when it comes to the type lighting that can be installed in a barn. Whatever you choose, they should be cool burning. Some types of lights, like halogen and incandescent bulbs can generate a lot of heat, which could be a fire hazard in some circumstances. Fluorescent bulbs, whether compact fluorescent or tubes are popular. If you live where it gets very cold, they can take a long time to light up. The fixtures that you choose for your lights should be agricultural or commercial grade. Household fixtures won't stand up to the heat and cold, wear and tear that they'll be exposed to in a stable.
All bulbs should be enclosed so that horses can't either hit them or tamper with them. Cages will prevent horses from being able to reach the bulb, but won't prevent glass shards from falling through if a bulb breaks. Shatterproof lenses may be a bit more expensive, but a safer option. This also prevents the problem of chaff, cobwebs and other debris accumulating on the bulb until it's a fire hazard. Switches should be industrial grade and designed for easy access for the humans, but out of reach of the horses. All outlets in a stable should be GFICs and switches, and lighting fixtures should be moisture proof.
Lights need to be placed to reduce glare and shadows and bright enough that it is easy to see to work. If light fixtures are placed within stalls, they need to be positioned so a horse cannot hit it with their head. This may not be practical because of ceiling height. If this is the case, the light needs to be placed outside of the stall, and a reflector can be used to send light where it is needed.
All wiring should be rodent proof. While some coated wiring is less appealing to rodents, it's wisest to use either metal sheathed wire, also called BX or armored wire, or conduit in barns. Some types of conduit can rust. An experienced licensed electrician is your best resource when planning the type of lighting and wiring to be used.
As you plan your stable's lighting, don't forget exterior lights. Well-placed exterior lights mean you won't be fumbling around in the dark getting to the door or finding the door latch. A timer or motion sensor installed in an over the door light is very handy. Be sure when designing your lighting system that you make light switches handy to the entrance door, so you don't have to grope your way in the dark to find them. A floodlight that shines into nearby pastures or paddocks is very handy when you have to find horses after dark.