The availability of energy resources likely affects
The availability of energy resources likely affects the body’s ability to expend effort. One means by which energy is released is through glucose metabolism. Glucose utilization as reflected by plasma glucose levels was reduced during the 24h sleep deprivation period and was associated with decreases in a composite of self-reported alertness items including a measure of subjective effort. The greatest decreases in Natural Product Library glucose utilization were found in those who felt least alert across the night. Changes in glucose levels were not correlated with vigilance performance, however . In another study, sleep deprivation with 48h with continuous work had no effect on resting blood glucose levels, however, participants perceived increases in imposed work load and reduced their walking pace .
Cardiovascular activity makes energy available throughout the system. Systolic blood pressure has been suggested to reflect increased effort. In a series of studies by Gendolla and colleagues, improved performance on tasks rated as difficult was positively correlated with changes in systolic blood pressure (SBP) and with the personal meaningfulness of the task . In addition, insomniacs, those reporting chronically insufficient sleep, showed higher systolic blood pressure reactivity during a memory task than non-insomniacs, suggesting that those who get insufficient sleep apply greater cardiological effort when performing on tasks . Other researchers who assessed heart rate reactivity in response to performance during sleep loss found phasic heart rate deceleration in persons who experienced 32h of sleep deprivation and received feedback for their reaction time performance (Steyvers, 1987). However, neither fatigue nor anticipated effort has been found to be predictive of cardiovascular reactivity .
Perception The increased reports of fatigue and changes in the perception of task difficulty with sleep deprivation may result from a finely-tuned mechanism by which the organism assesses its state, determines the resources needed to complete the task, and decides whether those resources are available or accessible or whether those resources need to be used or conserved for tasks of higher priority. When deprived of sleep for 48h and required to perform continuous work, participants perceived that the work load had increased and reduced their walking pace in response . Similarly, in a study of physically fit athletes, researchers found an increase in perception of task difficulty following fatigue . Adolescent athletes who had insufficient sleep reported poorer mood and considered their drills in sports practice to be more difficult, and those with frequent awakenings avoided the most challenging exercises .
Objective effort with sleep loss
Subjective effort with sleep loss
Individual differences Hockey et al.  found that while some participants reported exerting less effort, most reported expending more effort when sleep deprived than when fully rested. In a study of two medical residents monitored for physiological and effort over a three month period, one resident’s report showed no relationship between subjective effort and cortisol, and adrenaline and reported low workload, high levels of support and control, low levels of fatigue and anxiety and high levels of positive affective states. In comparison, data collected from the second resident showed relationships between subjective effort and cortisol, between subjective effort and noradrenaline, and perceived work demands, fatigue, subjective effort and adrenaline . One marker of vulnerability to sleep deprivation, a decrease in left parietal activation, has been correlated with within subject variability of reaction times . Genetic differences may account for the differences in vulnerability to sleep deprivation and the ability of the system to compensate for the limitations caused by sleep loss. A comparison of three mice strains found differences in the distribution of sleep and the time course of slow wave activity in response to sleep deprivation . In addition, species differences in attention, learning ability, memory and cortical responsiveness to exogenous substances suggests unique genetic sensitivities to stress on the system’s capacity. For example, in rats attention and memory behavior  and learning to press a lever for food when amphetamine injected into the nucleus accumbens  differs depending on the species. Mice show species-specific responses to scopolamine with regard to speed of response and spatial recognition memory . Such strain differences support the genetic basis of sleep loss vulnerability and suggest the existence of genetically determined differences in effort expenditure in response to sleep loss.