We had great results when we hung a light fixture in our coop. I have it on a timer, and since adding the light the chickens have been laying MUCH MORE consistently. For the times, check your local area's earliest/latest sunrise and earliest/latest sunset through the year. We set the timer to come on at the earliest sunrise time and stay on 30 minutes past the latest sunrise time (there will be a big difference between winter and summer), then it comes back on 30 minutes before the earliest sunset time and stays on until the latest sunset time.
Example: your area's sunrise ranges from 6:00am at the earliest and 8:00am at the latest
your area's sunset ranges from 6:00pm at the earliest and 8:00pm at the latest
You'd set the timer for the lights be on from 6:00 - 8:30 am and from 5:30 - 8:00pm.
Don't leave the light on continuously because the hens need rest just like we do.
From a more natural standpoint however, you may wish to let the birds get that extra rest and leave it alone. You'll have to buy some eggs in the fall/winter, but you won't have to install lighting and you'll be allowing nature to take its course. We choose artificial light, but it's a personal decision really.