The National Sleep Foundation recommends seven to nine hours of shuteye per night for adults between 26 and 64. Using functional MRI, researchers at Japan’s National Institute of Mental Health in Tokyo have shown why getting less often leads to bad moods and poor emotional health:

The sleep debt diminishes resting-state functional connectivity between the amygdala and medial prefrontal cortex.

Yuki Motomura, PhD, and colleagues report the results of their small study in Sleep.

The team recruited 18 healthy men to sleep as normal for two nights as a study control (average time in bed, nine hours) and then go two days on only three hours of sleep.

On day two of each session, the participants were imaged with fMRI. Immediately after the scanning, they filled out the state portion of the State-Trait Anxiety Inventory (STAI-S).

State anxiety items gauge anxiety levels by having subjects self-grade on such items as “I am tense,” “I am worried,” “I feel calm” and “I feel secure.”

The researchers found functional connectivity between the amygdala and medial prefrontal cortex significantly decreased—and STAI-S scores significantly increased, indicating higher anxiety and/or anger—when the participants were in a state of sleep debt vs. fully rested.

The team also observed significant correlations between reduced rapid eye movement (REM) sleep and reduced functional brain connectivity in sleep debt vs. the control state.

“These findings suggest that reduced medial prefrontal cortex functional connectivity of amygdala activity is involved in mood deterioration under sleep debt, and that REM sleep reduction is involved in functional changes in the corresponding brain regions,” the authors conclude. “Having adequate REM sleep may be important for mental health maintenance.”