Behavioral and psychological symptoms of dementia (BPSD) is a concept proposed by the International Psychogeriatric Association that encompasses aggression, restlessness, apathy, excitement, depression, hallucinations, delusions, anxiety, and depression, all frequently found in patients with dementia. BPSD is an important clinical condition that impairs patients’ daily lives and associated with relative’s care burden, early admission to a care facility, cognitive impairment, and so on. BPSD is thought to appear due to the progression of cognitive dysfunction (memory, disorientation, etc.) associated with the underlying dementia, and psychological and social factors are strongly related. In previous research, these psychological and social factors were regarded as stressors, and the relationship between BPSD and stress evaluated using salivary cortisol. As such, we would assume that BPSD is likely to manifest when stress matches the progression of cognitive dysfunction. Under stress, sympathetic and parasympathetic nerve activity are overactive, which compromises homeostasis. Stress indicators typically used imply measuring cortisol levels in the blood and saliva. However, in recent years, the reliability and validity of stress assessments that measure autonomic nervous activity by heart rate variability analyses has been reported.