Female hormone changes during menopause can affect the autonomic nervous system, circadian rhythm, and secretion of cortisol/melatonin, resulting in a vulnerability to insomnia. In this light, therapy has been gaining attention as a way to reduce stress hormones by stabilizing the autonomic nervous system. Thus, this study aims to objectively and scientifically analyze the impact of forest therapy in postmenopausal insomnia patients. The forest therapy program lasted 6 days, wherein 35 postmenopausal women performed activities such as trekking, leg massages, stretches, and bathing in warm and cold water. They also underwent serologic tests, participated in polysomnography (PSG), and answered sleep questionnaires before and after the program. Further, a statistical analysis compared the results. Serologic tests showed a significant reduction of cortisol from 10.2 ± 3.79 to 7.75 ± 2.81, while PSGs showed how sleep efficiency increased to 89.3 ± 4.3% (p < 0.01), and how waking after sleep onset reduced to 47.4 ± 22.3 min (p < 0.01). The total sleep time also increased to 428.5 min and sleep latency was 11.1 ± 11.0 min. Despite its limitations, forest therapy could be a good alternative to nonpharmacological treatment for mitigating insomnia in postmenopausal women.