Bike lights can be powered by battery or dynamo. New technology means that reliability and brightness are no longer a problem ... so there is no excuse for any cyclist to be riding without lights when it is dark. The lighting regulations change fairly often, but this is the latest situation. Note that if you ride a bike with lights that don't meet the UK regulations and you are involved in a crash, a smartbottom lawyer could question your choice as he/she tries to pass on blame or reduce compensation.
Battery lights range from cheap and cheerful to blindingly expensive. LED lights are the most common but be aware that these are often highly directional, so make sure that they are facing the traffic rather than pointing up or down.
Dynamo lights are making a comeback on mid-price bikes, especially when the dynamo is incorporated in the wheel hub. These are generally very reliable.
When riding in winter, make sure that your coat doesn't cover your light: it's done easily enough!
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