Strings of lights do not harm a tree unless it is brought inside in a pot and exposed to warm dry air and then suddenly taken back outside into the cold, or unless the lights are actually hot enough to cause burns.
Light does affect the growth cycle of trees, but chiefly in combination with temperature changes; the two are closely related in the natural world.
In the winter, the outdoor cold has more effect on a dormant deciduous tree than light does.
Leafless groves hung with small white lights, like the famous one at Tavern on the Green in Central Park in New York City, stay dormant because there are no leaves to absorb the light.
Light will riot affect the trees until they are also exposed to the warmer temperatures of spring.
Evergreens will react somewhat to extra light in winter, but they are also more sensitive to temperature, and their processes, like photosynthesis, proceed all year long at a much lower rate than for deciduous trees, with very slow uptake of water and transpiration.
That’s why they’re evergreens.