Heat rises, meaning it travels almost vertically. There is also heat effect by proximity and radiant heat.
Your monitor is likely vented near the top portion and is cooling by heat's natural tendency to rise out.
What matters is the style and wattage of the bulb to get an idea of how much heat it might give off,
and also how close it is to the back of the monitor. If it's an old incandescent bulb at 100 watts, and you
put it 1" away from the back of the monitor, it's a problem..the radiant heat would be insane (put your hand there).
An LED light throws way less heat (put your hand there).