Consciousness is the awareness of stimuli (both internal and external) and events. Awareness, however—both of internal psychological functioning and of external environmental factors—can change, and responses can vary widely as a result of an individual's level of consciousness. Imagine, for example, the response of a wide‐awake person to a knock on the door compared to that of a drunk person or one waked from a deep sleep. All living creatures undergo changes in levels of awareness throughout a 24‐hour period. Examples are the sleep‐wake cycles and the changes in biological cycles called circadian rhythms. Consequently, to understand the science of behavior and mental processes, psychologists must also understand the effects of levels of awareness (consciousness) on psychological functioning. Early investigations of consciousness. Psychologists have been concerned with the study of consciousness since Wilhelm Wundt conducted studies on the subject at the University of Leipzig in 1879. William James in 1890 described the mind as a stream of consciousness, a continuous flow of changing images, sensations, feelings, and thoughts. (To illustrate for yourself the concept of stream of consciousness, simply keep track of all your thoughts and feelings for 5 minutes, particularly while in class.) Edward Titchener (1867–1927) used a technique called introspection, whereby individuals could be trained to analyze conscious experiences. Sigmund Freud (1856–1939) was particularly interested in the lack of consciousness, the unconscious. Freud believed that the unconscious is a reservoir of unacceptable thoughts, wishes, and feelings that are beyond conscious awareness and that can be reached by psychoanalytic techniques.