noun
disheveled hair; tousled hair
Describes hair that is messy, tangled, or in disarray, often with a poetic or literary feel.
風で乱れ髪になった。
My hair got tousled by the wind.
彼女は乱れ髪のまま現れた。
She appeared with her hair disheveled.
寝癖 refers specifically to bedhead or hair messed up from sleeping, while 乱れ髪 is a more general term for disheveled hair.
Compound of 乱れ (midare, 'disorder') + 髪 (kami, 'hair'), with rendaku voicing of 髪 to がみ.