Dr. Richard Beyerlein MD, an OB/GYN in Eugene OR, explains that the most common cause of a change in period cycle length is stress and anxiety. This can be caused by any number of life events such as job changes, financial issues, or even personal relationships. When your body is under stress, it can cause hormonal imbalances that can affect your cycle length. In addition to stress, hormonal changes can also cause a change in period cycle length. Hormonal shifts can be caused by things like puberty, menopause or pregnancy. When your body goes through these natural transitions, it can throw off the balance of hormones and cause irregularities in your cycle. Finally, lifestyle changes such as dietary shifts and exercise routines can also cause a change in period cycle length. For example, if you start eating fewer calories or exercising more, it can throw off your hormonal balance and result in a longer or shorter menstrual cycle.