Circadian rhythms are crucial for regulating physiological and behavioral processes. Pineal hormone melatonin is often used to measure circadian amplitude but its collection is costly and time-consuming. Wearable activity data are promising alternative, but the most commonly used measure, relative amplitude, is subject to behavioral masking. In this study, we firstly derive a feature namedcircadianactivityrhythmenergy (CARE) to better characterize circadian amplitude and validate CARE by correlating it with melatonin amplitude (Pearson’sr= 0.46,P= 0.007) among 33 healthy participants. Then we investigate its association with cognitive functions in an adolescent dataset (Chinese SCHEDULE-A, n = 1703) and an adult dataset (UK Biobank,n= 92,202), and find that CARE is significantly associated with Global Executive Composite (β= 30.86,P= 0.016) in adolescents, and reasoning ability, short-term memory, and prospective memory (OR= 0.01, 3.42, and 11.47 respectively, allP< 0.001) in adults. Finally, we identify one genetic locus with 126 CARE-associated SNPs using the genome-wide association study, of which 109 variants are used as instrumental variables in the Mendelian Randomization analysis, and the results show a significant causal effect of CARE on reasoning ability, short-term memory, and prospective memory (β= -59.91, 7.94, and 16.85 respectively, allP< 0.0001). The present study suggests that CARE is an effective wearable-based metric of circadian amplitude with a strong genetic basis and clinical significance, and its adoption can facilitate future circadian studies and potential intervention strategies to improve circadian rhythms and cognitive functions.