Fatigue and mood swings are often linked for a number of different reasons. In some cases, fatigue simply causes mood swings and increases irritability; this is common among people who are very stressed, or not getting enough sleep at night. In other cases, fatigue and mood swings occurring together can be indicative of another mental or physical condition, such as depression, or a thyroid condition. Women who are pregnant will also frequently experience fatigue and extreme mood swings, more frequently in the first trimester, but are capable of occurring at any time throughout the pregnancy.
In many cases, the link between fatigue and mood swings is pretty direct. Not getting enough sleep at night, or experiencing high levels of stress every day, can increase levels of stress hormone in the brain. This can lead to feelings of anxiety or irritability, and people often respond to these feelings with mood swings. In general, trying to get more sleep at night and cuting back on stress as much as possible will also help to prevent the mood swings. Some people find that adding regular exercise to their day, though it may seem counter-intuitive, can actually help to reduce fatigue and mood swings.
If there isn't a simple explanation such as lack of sleep or stress contributing to fatigue and mood swings, it might be a medical condition. Mental illnesses such as depression or bipolar disorder can both cause fatigue and extreme mood swings. Treatment from a doctor and/or a therapist may be necessary to begin to resolve these problems. Other medical conditions can also cause fatigue and mood swings to occur. For instance, an underactive or overactive thyroid can cause an individual to feel either very tired all the time or uncomfortably energetic, and can also lead to frequent and unpredictable mood swings.
Another common cause of fatigue and mood swings is pregnancy. In fact, some women find that this is one of the first symptoms of pregnancy, and that it is similar to a severe version of PMS. It is common for women to feel extremely tired and to be very irritable or easily upset during the first few months of pregnancy. Some find that these symptoms will lessen after a while, while others experience them throughout the whole pregnancy. It is very difficult to predict, and there isn't much to be done about it, though some women find that getting regular exercise also helps to lessen these symptoms.