Non‐technical summaryNormal wakefulness relies on brain cells called orexin/hypocretin neurons. Activity of these cells stimulates awakening while their loss produces the sleep disorder narcolepsy. By studying what makes orexin/hypocretin cells more or less active, we can thus gain insights into how the brain switches between different states of consciousness. We describe a new way to turn orexin/hypocretin cells off using a chemical called glycine. We show that glycine shuts down the electrical activity of orexin/hypocretin neurons from the adult brain, but has the opposite effect in the very young brain. Apart from these direct actions on orexin/hypocretin cells, glycine also enhances the ability of other nerve cells to communicate with orexin/hypocretin neurons. These data shed new light on the basic chemical and physical mechanisms regulating orexin/hypocretin neurons, which may also be useful in improving therapeutic strategies for disorders such as insomnia.