HOUSTON, Texas (CNN) - Rob Sandler comforted his infant son as he lifted him out of the crib, cooing in his ear while he walked to the living room.

If his baby had needed to be soothed three months ago, Sandler, 36, of Houston, Texas, might have handed the baby off to his wife and then found an excuse to leave the house.

"Honestly, it felt like when I was at home, the walls became very, very close in. I wouldn't say claustrophobic, but very cabin feverish," Sandler said.

It turned out that Sandler, a medical device salesman, had more than cabin fever. He recently got an official diagnosis: He has a male version of postpartum depression.

"This comes as quite a shock to men who are expecting this wonderful time of baby bliss with the new baby and a time of bonding," said Will Courtenay, a San Francisco, California-based psychotherapist and founder of Saddaddy.com, who is a leading expert in the United States on paternal postnatal depression.

"Each day in the U.S., 1,000 new dads become depressed, and according to some studies that number is as high as 3,000. That's as many as one in four news dads who become depressed."

"We hear this from a lot of men," Courtenay said. "They can't stand to be around their baby...they can't stand the smell or the sound of their child screaming." Health Minute: Watch more on dads and postpartum depression »

Courtenay said a number of factors may cause the depressed feelings. "It's likely that sleep deprivation plays a major role," he said.

"Hormones may also play a role." According to Courtenay, "It's a double whammy. Not only do our testosterone levels go down, but our estrogen levels go up and these female hormones coursing through our body can really wreak havoc on a man's functioning." Although experts aren't exactly sure, they theorize that these fluctuations may be similar to "sympathy pains" that individuals feel when someone they're close to is hurting.

Male postpartum depression is different from the "daddy blues," he said. The signs of full-blown depression are usually more severe and last longer.

Some of the symptoms of postpartum depression are the same as those for generalized depression, such as sadness, a sense of worthlessness and a loss of interest in sex or hobbies.

A rocky relationship with a spouse, a sick or colicky baby, anxiety about becoming a father and a history of depression can also contribute to the condition.

Courtenay mentioned one of the best predictors of whether a man will become depressed is if his spouse is depressed. "Half of all men whose partner has postpartum depression are depressed themselves," Courtenay said.

Men are also more likely to hide their depression from loved ones, he added.

Sandler's symptoms began not long after his son Asher's birth in June.

"Something didn't feel right," he remembered. "I felt like a lot of loss of control of my life...a feeling of trappedness came over me and it would not lighten up. It kept getting worse and worse as the days went on."

Sandler said he would make excuses to get out of the house. "Simple errands to the store that would maybe last 30 minutes were now lasting an hour," he recalled.

Sandler's wife Traci, 38, who was recovering from a Caesarean section at the time, recognized something was wrong almost immediately.