Fifty percent of the total mass of the atmosphere is located in the lower 5.6 km (18,000 ft) of the troposphere. It is primarily composed of nitrogen (78%) and oxygen (21%) with only small concentrations of other trace gases.
Having irregular periods is very common. I see them in my practice all the time. The causes can range from something insignificant to something that requires treatment. For example, if you're a teen, your body's hormones can fluctuate and take some time until they find a balance.
Yes it can. In fact, I read somewhere that stress is one of the most common causes of irregular periods. It can be avoided though with certain measures. If stress is affecting your menstrual cycle too much, then consult an expert and move further with their guidance.