Emotion describes the coherence among affect (subjective feelings), physiological arousal, facial expressions, action readiness, and behaviors. These responses evolved to cope with environmental challenges that threatened survival. Analysis of language, emotion situations, and facial expressions uncovered basic emotions: anger, disgust, fear, happiness, love, sadness, surprise, and self-conscious emotions. The brain uses cognitive information from an emotional situation and from the body's physiological arousal to construct an emotion. Arousal also indicates the readiness of the body to act to satisfy the aim of an emotion. The strength of a challenging stimulus determines the intensity and duration of an emotion. Moods are a milder and more enduring form of affect. Stimuli responsible for mood occur gradually, such as hour of the day, day of the week, music, the seasons, and sleep.