Research Article Heart Rate Variability Reflects Self-Regulatory Strength, Effort, and Fatigue Suzanne C. Segerstrom and Lise Solberg Nes University of Kentucky ABSTRACT—Experimental research reliably demonstrates that self-regulatory deficits are a consequence of prior self- regulatory effort. However, in naturalistic settings, al- though people know that they are sometimes vulnerable to saying, eating, or doing the wrong thing, they cannot ac- curately gauge their capacity to self-regulate at any given time. Because self-regulation and autonomic regulation colocalize in the brain, an autonomic measure, heart rate variability (HRV), could provide an index of self-regula- tory strength and activity. During an experimental ma- nipulation of self-regulation (eating carrots or cookies), HRV was elevated during high self-regulatory effort (eat carrots, resist cookies) compared with low self-regulatory effort (eat cookies, resist carrots). The experimental ma- nipulation and higher HRV at baseline independently predicted persistence at a subsequent anagram task. HRV appears to index self-regulatory strength and effort, making it possible to study these phenomena in the field as well as the lab. A number of problems of modern life stem from failures of self- regulation, the ability to control one’s own thoughts, emotions, and behavior. Failure to inhibit the impulse to eat is in part responsible for obesity, for example, and failure to control anger can result in interpersonal aggression. Many such failures are unplanned; that is, they are lapses of self-regulation rather than intended acts. Why do people act contrary to their intentions? Experimental evidence indicates that self-regulatory strength— the ability to meet self-regulatory demands such as inhibiting impulses, making decisions, persisting at difficult tasks, and controlling emotions—is a resource that can be spent. In a series of studies, expending self-regulatory strength by eating un- desirable foods in the presence of desirable foods, making choices, and suppressing thoughts or emotions created subse- quent deficits in persistence at and performance on cognitive and motor tasks (Baumeister, Bratslavsky, Muraven, & Tice, 1998; Muraven, Tice, & Baumeister, 1998). Self-regulatory strength may therefore be analogous to muscle strength: The more effort is expended, the more the self-regulatory ‘‘muscle’’ is fatigued, and the less strength remains for further efforts (Mu- raven & Baumeister, 2000). This fatigue can lead to lapses of self-regulation. A difference between muscle fatigue and self-regulatory fa- tigue, however, is that people are not aware of self-regulatory fatigue. People do not report substantial differences in their appraisals of tasks that require self-regulatory effort versus tasks that do not, nor do they report differential affective conse- quences of those tasks (Baumeister et al., 1998; Muraven et al., 1998). Without an index of self-regulatory strength and fatigue—an electromyograph for the self-regulatory muscle— the concept of self-regulatory strength is difficult to apply empirically to naturalistic settings, in which a failure of self- regulation (e.g., eating a doughnut while on a diet) could be construed both as the indicator of self-regulatory fatigue and as the consequence of that fatigue. Unraveling this tautology re- quires the ability to measure how much self-regulatory strength people have available, how much self-regulatory effort they are expending, and whether they are vulnerable to the negative consequences of self-regulatory fatigue. A measure of parasympathetic control over the heart, heart rate variability (HRV), offers just such an opportunity. Para- sympathetic input to the heart via the efferent vagus nerve affects heart rate acceleration and deceleration related to res- piration: The heart speeds up after inspiration and slows down after expiration. More parasympathetic input results in more pronounced acceleration and deceleration and more variable intervals between heart beats, that is, higher HRV. HRV is a Address correspondence to Suzanne C. Segerstrom, Department of Psychology, University of Kentucky, 115 Kastle Hall, Lexington, KY 40506-0044, e-mail: scsege0@uky.edu. PSYCHOLOGICAL SCIENCE Volume 18—Number 3 275 Copyright r 2007 Association for Psychological Science