“… some (men) go through what’s called the male PMS: ‘IMS’ (Irritable Male Syndrome). This can be attributed to men experiencing a drop in testosterone , the hormone that gives them their mojo. Their IMS can happen at anytime, as testosterone levels fluctuate during the day, but they tend to be highest during the morning and drop as the day goes on.”

Stress precipitates IMS, as does the natural ebb of testosterone throughout the day or month (which you can somewhat reign in with a healthy diet and enough exercise).

Science compares guys to grumpy hormonal sheep, calls it Irritable Male Syndrome | Hot Topics - seattlepi.com