Imagine being in a sauna for a few hours. Then imagine getting out of it and walking straight into a giant freezer for another few hours. Do this several times a day and continue the routine for a couple of months. Some people say that's what spending summer in Japan is like.

Japan's summer months are notoriously hot and muggy, which can lead to a range of health problems. Natsubate (summer fatigue) leaves people feeling tired, lethargic and/or sleep-deprived. Many people lose their appetite and become irritated, while others suffer digestive problems such as diarrhea, constipation and/or giddiness.

Originally referring to a condition brought on by prolonged exposure to the sweltering summer heat, these days natsubate starts early even in late June for some people, because of sudden changes in the weather and freezerlike air conditioning in trains, buses and buildings.