Happiness is a subjective thing. To some, it might be embodied in healthy, caring and supportive relationships. To others, it might mean having nice things and the time to enjoy them. Or maybe it's a sense of being grounded and having the capacity to bounce back and thrive in spite of the pitfalls which can be part of merely being human. Regardless of your definition, the happiest people you know possess mental habits which differ markedly from those who tend to exude negativity. Here's what science says on the subject.

1. They spend very little--if any--time on Facebook.

Countless studies have been conducted regarding how its use affects people. In fact, researchers have found that the more you use Facebook, the worse you feel and that quitting the social network actually leads to higher levels of well-being.

2. They don't overshare mushy photos and updates about their mate.

According to a study published in the Personality and Social Psychology Bulletin, people who overshare about their love life on Facebook actually may have a weaker relationship. "[O]n days when people felt more insecure about their partner's feelings for them, they posted more about their relationships on Facebook than usual," the authors write.

3. They get adequate quality sleep.

A researcher at the University of Warwick in the United Kingdom found that improving your sleep quality and quantity is as good for health and happiness as winning the lottery. According to researchers, everyone on the planet should be logging at least seven hours of sleep a night. Here's how to get more of it.

4. They don't tell lies--even little white ones.

A University of Notre Dame researcher found that when people reduced the number of lies they told over 10 weeks they reported significantly improved physical and mental health.

5. They give their time and money away.

Americans who rate themselves as "very happy" volunteer an average of 5.8 hours per month, compared with those who describe themselves as "unhappy" and log only about 0.6 hours. That's according to researchers at the University of Notre Dame who also found that people who donate more than 10 percent of their incomes are less depressed than people who give less. Being generous emotionally also pays off--people who are available and hospitable to others are significantly more likely to be in excellent health.

6. They're more physically active.

While countless studies have linked intentional exercise with increased health and happiness, researchers at the University in Cambridge found a correlation between mere movement and happiness. Study participants tended to rate themselves as happier if they had been walking or performing other non-rigorous activity--essentially anything more than sitting or lying down.

7. They make enough money.

The maxim that "Money can't make you happy" isn't necessarily true. Princeton researchers have determined that subjective well-being rises with income, but only up to $75,000. After that point making more money doesn't result in greater happiness. And--as anyone who's ever been broke can attest--low income can be correlated with low life evaluation and low emotional well-being. So, while making millions may not make you happier than a moderate income, being poor certainly can make you unhappy.

8. They are grateful.

Researchers at the University of Miami conducted a study during which they asked three groups of participants to write about certain topics each week. One wrote about things that happened which they were grateful for. Another wrote about negative events or things which had irritated them throughout the week. Another wrote about events which had affected them without any focus on whether they were positive or negative. After 10 weeks, the people who had written about things they were thankful for reported being more optimistic and feeling better about their lives. They also exercised more and visited their physicians less than the participants who had focused on things which annoyed them.

9. They focus on the here and now.

It's hard to be happy when you're ruminating about the past or worrying about the future. In fact, the happiest people concentrate on the present moment. That's according to happiness researcher Matt Killingsworth who built an iPhone app people used to report their feelings in real time. The app sent them signals throughout the day and asked them questions about what they were doing just before receiving the signal. His data, which was compiled from 650,000 real time reports from more than 15,000 people, showed that people are significantly less happy when they let their minds wander, compared to when they focus on what they're currently doing, even if what they're doing is something mundane like commuting.

"I think part of the reason, a big part of the reason, is that when our minds wander, we often think about unpleasant things, and they are enormously less happy when they do that, our worries, our anxieties, our regrets," he said at a TEDxCambridge event. "And yet even when people are thinking about something neutral, they're still considerably less happy than when they're not mind-wandering at all. Even when they're thinking about something they would describe as pleasant, they're actually just slightly less happy than when they aren't mind-wandering."