Non‐technical summary  A growing body of evidence has shown that energy status has a significant impact on the behavioural states in animals and that the availability of nutrients (energy state) in the brain may modulate animal behaviours. In this study, we report that the intracellular energy stores determine activity in a selective group of nerve cells (hypocretin‐containing neurones) in the brain. The unique energy state of hypocretin neurones correlates with behavioural states of animals, i.e. the energy level is low during sleep and high during wakefulness. These results suggest that hypocretin neurones may act as an ‘energy gauge’ in the brain, which integrates nutritional, energetic and behavioural signals critical for the survival of animals.