Aggressive behaviour reaches highest levels at sunset, finds body-clock study

Aggression in dementia could be treated by targetting brain region running internal body-clock, say scientists

Elderly people with diseases like dementia become more aggressive in the evenings (Getty Images/iStockphoto)

Aggressive behaviour reaches its peak at sunset according to researchers who have isolated the part of the brain responsible for both our internal clock and mood.

Early evening aggression is a common phenomenon in people with dementia, known as “sundowning”, and the US team say this may be down to impairment of this biological circuitry.

Dictated by changes in light throughout the day and season, the 24-hour circadian cycle is known to influence a range of biological functions including emotion, body temperature, digestion and hormone levels.

Now Harvard Medical School researchers have shown these circuits in the brains of mice also drive aggression, which peaks at the end of the day and is at its lowest first thing in the morning.

This could open the door for new treatment of neurological diseases to relieve distress and make behaviour more manageable, according to the authors of a paper published in Nature Neuroscience.

“Sundowning is often the reason that patients [with dementia] have to be institutionalised, and if clinicians can control this circuit to minimise aggressiveness at the end of the day, patients may be able to live at home longer,” said senior author Dr Clifford Saper, chair of the Department of Neurology at Beth Israel Deaconess Medical Centre.

Dr Saper and colleagues studied how male mice responded to intruders in their territory throughout the day, and found aggressive attacks increased as the day wore on.

“The mice were more likely to be aggressive in the early evening around lights out, and least aggressive in the early morning, around lights on,” Dr Saper said. “It looks like aggressiveness builds up in mice during the lights on period, and reaches a peak around the end of the light period.”

They then used genetic tools to inhibit a region in the mouse brain known to regulate their circadian clock, and were able to stop this cycle of increasing and decreasing aggression.

Instead, the mice who had impairments to their circadian clock were more aggressive all through the day, and spent significantly more time attacking intruders.

Stimulating this region of the brain, as in normal operation of the circadian circuitry, cools off aggression and could have implications for people.

“Our results in mice mimic the patterns of increased aggression seen in patients during sundowning,” Dr Saper said. “This new research suggests this pathway may be compromised in neurodegenerative diseases. Examining changes to this pathway in patients could provide insight into future interventions that could greatly improve the quality of life for patients and caregivers alike.”