Sleep disorder refers to any condition that significantly impairs the ability to fall asleep, stay asleep, achieve restorative sleep, or maintain normal sleep-wake patterns, leading to excessive daytime sleepiness, fatigue, mood disturbances, or impaired functioning. Common types include insomnia (difficulty initiating or maintaining sleep despite adequate opportunity), sleep apnea (repeated pauses in breathing during sleep, often due to airway obstruction, causing snoring, gasping, and fragmented rest), narcolepsy (sudden, uncontrollable sleep attacks and loss of muscle tone/cataplexy), restless legs syndrome (uncomfortable urges to move the legs, usually at night, with creepy-crawly sensations), periodic limb movement disorder (involuntary jerking of limbs during sleep), circadian rhythm disorders (e.g., delayed sleep phase, jet lag, or shift work disorder disrupting natural body clock timing), and parasomnias (abnormal behaviors like sleepwalking, night terrors, or REM sleep behavior disorder where people act out dreams). These disorders can be primary or secondary to medical conditions, medications, psychiatric issues, or lifestyle factors, often resulting in chronic tiredness, concentration problems, and increased risk for accidents, cardiovascular disease, or mental health issues.