A Flight Attendant reported that she called in fatigued after circadian rythm and decreasing rest time resulted in concentration; memory and wakefulness issues.

Narrative

Two-leg through flight; both legs of flight fatigued. Both Flight Attendants fatigued; took self off rest of day due to fatigue. I do not believe I actually broke any FARs; but if I continued trip I was compromising passengers as well as own safety. [I] did not want to set myself up for negative incidents of any kind. Experienced decreased concentration; decreased memory; difficulty staying awake; difficulty answering passenger's questions; headache; finally chills (common symptom when I am extremely overtired). Safety of flight was compromised. 1. Flight Attendant 'B' gave only a partial pre departure safety; I did not even notice. 2. Did not remember to brief exit seats occupants on either flight until moments prior to main cabin door closure chime. I kept thinking 'I am forgetting something'. 3. I had to actually pinch myself (inflicting pain on self) to keep awake on takeoff and landing (other incidences not related to safety occurred). Duty time increased as rest periods decreased. Circadian swaps became earlier as trip progressed. Long sit times second (and third days) also contributed to fatigue. (I was exhausted at end of second day... long sit in outstation; no where to sleep; exercise; shower.....then a circadian swap.) *Circadian swaps are a big factor in fatigue. I am unable to 'command' my body to 'sleep' and change its natural rhythms on command.