Neurons form complex network which work as functional circuit to regulate behavior in the brain. Little is known about how these circuit functions to regulate behavior since it had been impossible to control the activity of specific type of neurons in the brain. Recently developed techniques, optogenetics and pharmacogenetics (chemicogenetics) enable control the activity of specific type of neurons in the brain. These new techniques allow us to study the function of these network and behavior using the whole animal. In this symposium, I will show recent progress of our study about the physiological role of melanin concentrating hormone (MCH)-producing neurons in the hypothalamus. MCH neurons are located in the lateral hypothalamus. They project throughout the brain from there. Recent study revealed that MCH neurons are involved in the regulation of sled/wakefulness. To study the role of MCH neurons in the regulation of sleep/wakefulness, the activity of MCH neurons was manipulated by optogenetics. Activation of MCH neurons 3-fold increased time spent in REM sleep. However, inhibition of MCH neurons did not affect time in REM sleep. To further study the role of MCH neurons on the sleep/wakefulness regulation, temporally controled MCH neurons specific ablation was performed by using tTA/TetO gene expression system. MCH neurons specific ablation decreased time in non-REM sleep without affect on the time in REM sleep. Taken together, these results indicate that acute activation of MCH neurons is sufficient, but not necessary, to trigger the transition from NREM to REM sleep and that MCH neurons also play a role in the initiation and maintenance of NREM sleep.