There's a reason why they call work work--it's not easy, and it can take a lot out of us. We get tired, our minds start to flag during the course of the day, and we lose our focus and concentration.

According to Emma Seppala--science director for Stanford University's Center for Compassion and Altruism Research and Education--there are a number of things you can do to replenish your mental energy that don't involve drinking more coffee or high-caffeine sodas.

Her book The Happiness Track, Emma says, presents 4 scientifically proven ways you can restore your energy when it's depleted and you feel drained. Not only will you feel better, but you will be happier and more successful as a result.

1. Do something that makes you feel positive

Says Emma, "You know best the kinds of activities that you find uplifting. Create a list and keep it handy, so you don't have to figure out what to do when you feel mentally exhausted. For example, if you're at work, replenishing your energy might mean going for a walk, taking a break, watching a funny YouTube video, looking at pictures of your loved ones, meditating, or engaging in a random act of kindness for a colleague."

2. Remember the big picture

"Focus on the why," Emma suggests, "rather than on the how of a task or job. Understanding how your work connects to what you care about and to your values will restore your energy. For example, if your company sells a certain device or product and you value making a difference in people's lives, you can think about how that device or product is helping people fulfill their needs."

3. Practice gratitude

According to Emma, "Research has shown that feeling grateful helps you replenish your energy in the face of fatiguing tasks. Let's say you don't like your job. Regardless, there are always things that warrant being grateful: You have a job when others don't. The reason gratitude has such a replenishing effect is that feeling grateful both increases positive emotion and helps you see the big picture."

4. Detach from work when you're not working

"Many times people take work home with them at night or do it during their time off," says Emma. "As a consequence, the stress of the day blends into evenings and vacations and eats up recovery time. People who do not know how to detach from work during their off time experience increased exhaustion over the course of one year and are less resilient in the face of stressful work conditions. Psychological distance from work is the fastest path to recovery and leads--surprisingly perhaps--to increased productivity.