We know that sleep helps our body recover from the day and repair itself and that it helps us focus and be more productive. It turns out sleep is also important for happiness.

In NutureShock, Po Bronson and Ashley Merryman explain how sleep affects positivity:

Negative stimuli get processed by the amygdala; positive or neutral memories gets processed by the hippocampus. Sleep deprivation hits the hippocampus harder than the amygdala. The result is that sleep-deprived people fail to recall pleasant memories yet recall gloomy memories just fine.

In one experiment by Walker, sleep-deprived college students tried to memorize a list of words. They could remember 81% of the words with a negative connotation, like "cancer." But they could remember only 31% of the words with a positive or neutral connotation, like "sunshine" or "basket."

The BPS Research Digest explores another study that proves sleep affects our sensitivity to negative emotions. Using a facial recognition task throughout the course of a day, researchers studied how sensitive participants were to positive and negative emotions. Those who worked through the afternoon without taking a nap became more sensitive to negative emotions like fear and anger.

Using a face recognition task, here we demonstrate an amplified reactivity to anger and fear emotions across the day, without sleep. However, an intervening nap blocked and even reversed this negative emotional reactivity to anger and fear while conversely enhancing ratings of positive (happy) expressions.

Of course, how well (and how long) you sleep will probably affect how you feel when you wake up, which can make a difference to your whole day.

Another study tested how employees' moods when they started work in the morning affected their entire work day.

Researchers found that employees' moods when they clocked in tended to affect how they felt the rest of the day. Early mood was linked to their perceptions of customers and to how they reacted to customers' moods.

And most importantly to managers, employee mood had a clear impact on performance, including both how much work employees did and how well they did it.