Human herpesvirus (HHV)-6 and HHV-7 establish life-long latency, a hallmark of herpesviruses, reactivate frequently, and are shed in saliva. To investigate the viral reactivation, we have identified the latency-associated transcripts of HHV-6, and have revealed the partial mechanism of HHV-6 reactivation. HHV-6 established latency in the macrophage, kept a fairly stable intermediate stage between latency and reactivation, and the viral reactivation was induced by two or more factors. To identify the factor (s) of HHV-6 reactivation, we studied the association between HHV-6 reactivation and the work-induced fatigue in healthy adults. Reactivation of HHV-6 was examined for viral DNA by semi-quantitative PCR method. As a result, 88% of healthy adults shed the reactivated HHV-6 in the saliva during the fatigue, and 23% shed HHV-6 after holidays (approximately 1 week). The copy number of HHV-6 DNA was also reduced after holidays. In HHV-7, 52% of healthy adults shed the reactivated HHV-7 in the saliva during the fatigue, and 30% shed HHV-7 after holidays; however, there were no significant differences in their positive ratio and in the amount of viral DNA. These findings suggest that HHV-6 is reactivated during the work-induced fatigue, and HHV-6 reactivation can be an objective biomarker for fatigue.