The reason the hospitals take flowers of of hospital rooms at night is because at night the plants or flowers will use more oxygen which is thought to take the oxygen away from patients.
So the plants are removed from the hospital rooms at night so that the patient has enough oxygen through the night.
However some hospitals will leave plants and flowers in rooms at night but I've seen some hospitals remove the plants at night and then return them the following morning.
It's up to the hospital and their policies.
But it's perfectly fine to leave flowers in hospital rooms at night as the patient will not be harmed.
At night, this 'photosynthesis' is turned off and most plants use oxygen to turn the carbohydrates into new leaves and shoots (as they do all day as well).
Because of this 'respiration', patients in hospitals were once advised to take flowers and plants out of their rooms at night.
In fact, flowers add far more oxygen to a hospital room than they use.
In daytime, plants emit 10 times more oxygen than they use up at night, so a hospital room with flowers in it will actually end up more oxygenated than one without.