Any habit, whether good or bad, is always harmful when it is followed to the extreme. I have noticed that those who are too extreme about their health tend to fall sick more often and most often don’t enjoy their diet or routine as often. They tend to miss out on many aspects of life. It is their lack of flexibility that makes them more vulnerable to illnesses.
It is best to always follow a good routine with good habits, but at the same time to be flexible about your lifestyle. If you are an early sleeper and go to bed before ten, then one night of staying up late should not be harmful if you are flexible about it. I know people who are very rigid about what time they go to sleep and if they stay up late once, then they feel sick for a week. If you are an early sleeper, but understand that sometimes circumstances come up in which going to sleep early is not an option and you must adjust. If you willingly adjust to changes or disturbances in your diet and lifestyle, then one night of staying up late will not harm you in any way.
Furthermore, there is no need to be rigid about whether or not you are sitting next to a person with a cold in the movie theater. If you have a strong immune system and take good care of yourself, being in the same room as someone who has the flu or a cold will not make you sick.
