Sitting at your desk all day or on your sofa watching TV could make you stupid, scientists have suggested.
Researchers have discovered those with a sedentary lifestyle have a smaller brain region important in forming memories.
The study, by researchers at the University of California at Los Angeles, adds to a growing list about the dangers of sitting for too long.
An array of evidence has already linked the bad habit to heart disease, diabetes, several forms of cancer and an early death in recent years.
But the new research, derived from 35 participants, suggests sitting for too long could even boost the risk of dementia.
Those with the lazy lifestyles had less grey matter in the medial temporal lobe (MTL) - even if they went for regular brisk walks, cycle rides or jogs.
A decline in this area has repeatedly been shown to be an early warning sign of Alzheimer's disease in middle-aged and elderly patients.
The study, published in PLOS ONE, quizzed the volunteers, who were aged between 45 and 75, about their levels of exercise.
Each person underwent a high-resolution MRI scan which provides a detailed look at the MTL, an area involved in the formation of new memories.
The researchers, led by biostatistician Dr Prabha Siddarth, found that sedentary people had a thinner MTL.
Dr Siddarth and colleagues revealed how this finding remained true - even when volunteers had high levels of physical activity.
The researchers warned that the study 'does not prove too much sitting causes thinner brain structures'.
However, they were keen to add the research does prove 'that more hours spent sitting are associated with thinner regions'.
The researchers wrote: 'MTL thinning can be a precursor to cognitive decline and dementia in middle aged and older adults.
'Reducing sedentary behaviour may be a possible target for interventions designed to improve brain health in people at risk for Alzheimer's disease.'
The scientists, who were part funded by the US government, now plan to follow up the preliminary findings on a larger trial group.
This will help determine if sitting causes the thinning and what precise role gender, race and weight may play, they said.
Previous research shows physical activity correlates with higher volume in the hippocampus, a small, memory-critical region deep within the brain.
The temporal lobes are on either side of the brain, near the temples. They deal with memory, including recognition of faces and objects, and language.
The Alzheimer's Society said: 'Our day-to-day memory of personal experiences, known as episodic memory, is very closely linked to the hippocampus.'
They added that this is 'inside the temporal lobe on each side of the brain'.
'The outer part of each temporal lobe is where we store general knowledge, which is a different type of memory known as semantic memory,' a spokesperson said.
'The left temporal lobe usually deals with facts, the meanings of words and the names of objects. This lobe is central to understanding speech and talking.
'The right temporal lobe usually deals with visual material. This lobe is central to recognising familiar objects and faces. '