In two experimental studies, we investigated effects of sleep duration and light intensity on effort-related cardiovascular response. We predicted, that due to reduced alertness after short sleep and during exposure to lower light intensity, perceived task demand and thus mental effort should increase. Effort was operationalized as sympathetic activity of the autonomous nervous system and assessed through reactivity of the cardiac pre-ejection period (PEP) and systolic blood pressure (SBP). Thirty-nine (Study 1) and 24 (Study 2) healthy volunteers underwent two experimental sessions, one after 5 and one after 8 hours of sleep opportunity in a counterbalanced order. Light intensities were 1 lux vs. 65 lux (between-person) in Study 1 and 100 lux vs. 500 lux (within-person) in Study 2. Experimental lighting conditions were applied for 15 min which was followed by a 5-min auditory two-back (Study 1) or modified auditory Sternberg (Study 2) task phase. No significant effects of sleep duration or light intensity on effort-related cardiovascular response could be shown in Study 1. An exploratory median split on sleepiness ratings revealed a stronger PEP response during the 1st minute of task performance for participants reporting high sleepiness (p = .041). In line with our hypothesis, Study 2 showed a stronger SBP and DBP reactivity after sleep restriction (ps < .001), indicating higher effort investment. Contrary to our prediction, 500 lux light led to higher PEP reactivity compared to 100 lux (p = .032). Overall, our results provide the first evidence of sleep duration and sleepiness impact on mental effort.