Going out for lunch may be going out of style.

According to research from the NPD Group and first published in the Wall Street Journal, Americans made 433 million fewer trips to restaurants for lunch last year, resulting in a $3.2 billion loss for the restaurant business.

Meanwhile, people eating out for breakfast has gone up.

Sacramento State Professor of Finance, Dr. Sanjay Varshney, says there are several factors that have led to this trend.

"In general, I think people are getting healthier," Varshney said. "So their habits follow them in terms of how they actually spend their money on meals outside versus bringing them from home."

"Second," Varshney continued, "Many of the companies are actually providing employees with amenities. For example, fitness centers on the work site, and I think people are taking advantage of those amenities."

Varshney also said people are much busier now and would more people are choosing to eat lunch at their desk to save time.

And, he added, it has to do with millennials.

"Millennials are very careful in how they spend their money," Varshney said. "So for example millennials are more inclined to take public transportation. Millennials are not averse to living closer to downtown. Millennials prefer to rent than to buy. Millennials also sometimes prefer healthier choices when it comes to their breakfast options, lunch options and dinner options. And yes I think that millennials are more inclined to save that lunch hour and actually work out or have a healthier option right at their desk and then save the time in the evening for bigger and better things."