1 serotonin
Serotonin is responsible for maintaining appetite, sleep and, you guessed it, mood balance, but a deficit leads to depression. Bright light, exercise and good nutrition increase serotonin levels. But it has also been found that there may be a two-way link in which serotonin could improve mood, and mood could, in turn, increase serotonin* – meaning that thinking positively could cause your body to raise serotonin levels, helping you out in the happiness stakes.
2 oxytocin