1. Division of Endocrinology and Metabolism, Department of Internal Medicine, Faculty of Medicine, University of Tsukuba, Tsukuba, Ibaraki 305-8575, Japan
2. Division of Cardiology, Department of Internal Medicine, Faculty of Medicine, University of Tsukuba, Tsukuba, Ibaraki 305-8575, Japan
3. Department of Hematology, Endocrinology and Metabolism, School of Medicine, Niigata University, Niigata, Niigata 951-8510, Japan
4. Division of Endocrinology and Metabolism, Center for Medical Sciences, Ibaraki Prefectural University of Health Sciences, Ami, Ibaraki 300-0331, Japan
5. International Institute for Integrative Sleep Medicine (WPI-IIIS), University of Tsukuba, Tsukuba, Ibaraki 305-8575, Japan