-contemplating events that happened in the past, might happen in the future, or may never happen at all.
"Indeed, mind-wandering appears to be the human brain's default mode of operation," according to the study by Harvard University psychologists Matthew Killingsworth and Daniel Gilbert.
"A human mind is a wandering mind, and a wandering mind is an unhappy mind," Killingsworth and Gilbert said.
"The ability to think about what is not happening is a cognitive achievement that comes at an emotional cost".
The study is based on data from an iPhone app designed by Killingsworth to study happiness.
Data was collected from 2,250 participants aged 18 to 88 years.
The researchers used the iPhone app to gather 250,000 data points on their subjects' thoughts, feelings and actions as they went about their lives.
The participants were randomly asked to report how happy they were, what they were doing, and whether they were thinking about their current activity or something else that was pleasant, unpleasant or neutral.
On an average, respondents reported that their minds were wandering 46.9 per cent of the time, and no less than 30 per cent of the time during every activity except making love.
Killingsworth and Gilbert found that people were happiest when making love, exercising or engaging in conversation. They were least happy when resting, working or using a home computer.
Analysis of the samples over time showed that those whose minds wandered when asked about their condition reported being more unhappy after a mind-wandering episode than when focused on their current activity.
Out of the 22 activities that participants reported doing, including watching TV and shopping, making love had the least mind-wandering associated with it.
In all other activities like eating or walking, participants said their minds wandered more than 30 per cent of the time.
"Mind-wandering appears ubiquitous across all activities," Killingsworth, a doctoral student in psychology at Harvard, said.
"This study shows that our mental lives are pervaded, to a remarkable degree, by the nonpresent".