The Mid-Life Crisis -- a summary of points found so far:

From About.com:

A midlife crisis is experienced between the ages of 40 and 60. It was first identified by the psychologist Carl Jung and is a normal part of the maturing process. Most people will experience some form of emotional transition during that time of life. A transition that might cause you to take stock in where you are in life and make some needed adjustments to the way you live your life. Most seem to come through the process smoothly without making major life changes. For some, a midlife crisis is more complicated. It can be an uncomfortable time emotionally which can lead to depression and the need for psychotherapy. Those who have a hard time with this transitional stage might experience a range of feelings such as: 

• Unhappiness with life and the lifestyle that may have provided them with happiness for many years. 
• Boredom with people and things that may have been of interest to them before. 
• Feeling a need for adventure and change. 
• Questioning the choices, they have made in their lives and the validity of decisions they made years before. 
• Confusion about who they are and where they are going. 
• Anger at their spouse and blame for feeling tied down. 
• Unable to make decisions about where they want to go with their life. 
• Doubt that they ever loved their spouse and resentment over the marriage. 
• A desire for a new and passionate, intimate relationship. 

Most people who have a difficult time during midlife and go into crisis mode do so because of external factors. They may be experiencing stress in their life that makes the transition more difficult or they may have childhood issue that were never dealt with that come to the surface during this time. Some external factors that may cause this time in life to be problematic are: 

• Significant Loss: The death of a parent or family member can cause grief, which is difficult enough to come to terms with, without having to also cope with the feelings of a midlife transition. Put the loss of a loved one with the feelings that accompany midlife and the whole process becomes bewildering and overwhelming. 
• Avoidant Personality: If a person has a tendency to avoid conflict in their personal relationships, suffers from feelings of inadequacy, are emotionally distant [or] has low self – esteem they will find midlife transition harder to navigate. This personality type has a deep fear of feeling shame and rejection. Such feelings will keep them from seeking help should their emotions become overwhelming. More than likely, they will run from their problems instead of trying to find solutions to them. It’s this personality type that normal ends up in divorce court during midlife. 

Whether there are external factors that make the process more difficult or not, there is an internal process that is gone through. If a person lacks understanding of the process, he may find himself making irrational decisions he may later regret such as leaving a job, divorcing his spouse and throwing away the security that he built during the first part of his life. 

From WebMD:

She goes through menopause, which means both biological and psychological changes. The psychological changes a woman experiences at menopause can cause her to question how she has lived her life and whether she should make changes to the way she lives. 

From DivorceSupport: 

If your spouse is going through a midlife crisis, you can count on her doing things that will make you angry as hell. Lashing out at your spouse will help you feel temporarily better. It won’t change your spouse’s behaviors and will only lead to more conflict in the relationship. Get rid of your anger in a manner that means not engaging in conflict with your spouse. You may have had a wonderful marriage. You may have been a couple who discussed and worked through every problem as it came up. You are no longer that couple and you have to do a 180 and not expect your spouse to care about your relationship or talking about your relationship. If your spouse has distanced herself from you, insisting they talk about the relationship will only push them further away. Your spouse is going through changes that have caused her to lose interest in you and the relationship. That area of their life is low man on the totem pole right now. The more you attempt to discuss it the less interested they will become. It is human nature to want what we can’t have. Why should your spouse put any thought into a relationship with you if you are constantly reminding her how much you love [her] or want to save the marriage? You will get further with a midlife crisis spouse if at the same time they are distancing themselves from you, you distance yourself from them. So, instead of relationship talks, become mysterious, get a life, get out and make her wonder why you aren’t so focused on them. 

Your midlife crisis spouse needs therapy. Don’t expect that to happen! Your spouse is going to stubbornly think there is nothing wrong with her, that you are the problem and if not for you their lives would be smooth sailing. You are going to need a therapist. Someone who can listen to your concerns and help you work through the confusion caused by your spouse’s odd behaviors. 

From WebMD:

A midlife crisis might occur anywhere from about age 37 through the 50s, he says. By whatever term, the crisis or transition tends to occur around significant life events, he says, such as your youngest child finishing college, or a "zero" birthday announcing to the world that you're entering a new decade. "The death of parents can be a marker, too, for these midlife events," Jones says. 

Meanwhile, midlife women may become more selfish, Jones says, even though they value relationships. They may feel they have "paid their dues". 

In midlife, people need to be aware of symptoms of serious depression, such as: 

• Change in eating habits 
• Change in sleeping habits, fatigue 
• Feelings of pessimism or hopelessness 
• Restlessness, anxiety or irritability 
• Feeling of guilt, helplessness or worthlessness 
• Loss of interest in activities once enjoyed, including sex and hobbies 
• Thoughts of suicide or attempts at suicide 
• Physical aches or pains such as headaches or gastrointestinal upset that don't respond to treatment.