A study published in 2009 tested eighteen young healthy males between 19 and 31 years old. All participants were right-handed and had no history of a head injury, or of any medication that could affect a person central nervous system or endocrine system . All of the volunteers participated in two different sessions a month apart. The study consisted on the participants viewing movie clips and pictures that belonged to two different categories: neutral or negative. The participants had to memorize then rate each movie clip or picture by pressing a button with their right hand. They were also monitored in other areas such as their heart rate, pupil diameter, and stress measures by collection of saliva throughout the experiment. The participants mood was assessed by using the Positive and Negative Affect Schedule . 
However, chronic use of stimulants can be counterproductive. There are several reasons why. For one, it tends to stimulate the stress-response pathways of the body — so if you’re already stressed and have overactive stress response systems in your body, caffeine will stimulate those systems even more. Also, it can create too much release of stimulating neurotransmitters in the brain, which eventually leads to neurotransmitter resistance and imbalances of several neurotransmitter systems in the brain. (I go through the science on coffee and energy levels in this video).