Traditional incandescent Christmas lights do warm up under operation. Light from an incandescent bulb is actually created by heating that tiny little filament inside the bulb. Allowing airflow around the lights and not installing them on flammable materials are good safety measures and good common sense.
Wondering if your light string is incandescent or LED? A quick way to determine this is to plug them in and touch the tip of a bulb to see if it warms up a little.
LED Christmas lights do not warm up under operation so if your project requires that lights bulbs be concentrated together in a small space, they might be a better choice.
Filling a bottle with lights?
Many of our customers use incandescent craft light strings containing 10, 15 or 20 light bulbs to light wine bottles and they are super beautiful. We always suggest they not cap them off with a decorative stopper since that will hold in heat when they are plugged in and shorten the life of the light string and in some cases fade off darker colors like green and blue.
We also suggest that wine bottles filled with lights that warm up not be placed on vintage furniture – like a beautiful baby grand – and that they not be operated without supervision.