HeadcanonsGuardians rarely , if ever , need rest , Exos in particular . However , when they overexert themselves through overuse of abilities , dying too frequently , or exposure to Darkness , they suffer Acute Light Exhaustion . It manifests as heavy physical and mental fatigue , inability to use Light abilities , and in severe cases , long periods of a deathlike coma . When Guardians are reborn the first time , they are partially brainwashed by their Ghost to ensure their combat and survival skills . This is why they don ' t remember their past lives , and are able to use weapons and their abilities instinctively . They sometimes experience momentary flashbacks to their previous lives , but rarely , if ever , enough to be conclusive .