Summary: A new study reveals exposure to dim light might impact memory and learning. Researchers report rodents exposed to dim lighting lost 30 percent of hippocampal capacity and performed poorly on spatial tasks they had previously experienced.

Source: Dim Light May Make Us Dumber – Neuroscience News

The rats were kept in a bright, 1000 lux environment or a 50 lux environment. Supermarkets are on average only 750 lux to give you an idea on the brightness of the room. A 50 lux environment would be something like a public walkway at night.