From a thread on MLC which rings HOME to me and it is so true:

Depression sign #28: ML'ERS HAVE MAJOR MOOD SWINGS

This is a very obvious sign of depression but worth writing about. Family members who witness this depression sign often feel like they are going insane. The frequency of the mood swings with mid-lifers varies. Some experience rapid cycling, others much slower. Loved ones describe their mid-lifers as having Dr. Jekyl/Mr. Hyde personalities. They begin to feel like they are walking on egg-shells. The littlest thing can set the mid-lifer into a rage or period of depression. Some family members may feel their mid-lifer is on drugs. These mood swings may or may not affect the work environment. Some mid-lifers are better at controlling what they let others see. This therefore leaves the LBS feeling responsible for the mood swings and their world begins to fill with self-doubt.