Nootropics are drug or supplements that are purported to enhance cognitive function. Cognitive function can be augmented by increasing working memory, attention, creativity, logical problem solving, and other cognitive domains. Nootropics are not necessarily exotic compounds that you've never heard of. For example, coffee is a nootropic that enhances concentration and is used every day by more than 50% of adults in the United States. Coffee enhances attention and arousal by antagonizing adenosine receptors. Adenosine is an endogenous (naturally present) neurotransmitter that promotes sleep. Nootropics do not resemble the drug fictional drug NZT in the TV series Limitless. There are major limitations to nootropics. Most nootropics merely provide a modest boost to cognitive ability. Nootropics are broad in scope and include a variety of substances. For example, melatonin, l-tyrosine, and modafinil are all regarded as nootropics.