Darkness is required. Most Northern Lights locations are found at high latitudes, meaning there is no darkness from mid-April until mid-August. In this period of time, no Northern Lights can be observed.
In the most intense Northern Lights area (notably Alaska, Iceland, Northern Scandinavia and Yukon), the lights are observed from late August to mid April. However, from late September to late March, it is dark after 6pm, and one enjoys maximum chances.
On a yearly basis, the Lights are at their peak in September and March. The reasons for this trend are due to the March and September equinox.