When Goldilocks tasted the three bears' porridge, one was too hot, one was too cold, and one was just right.

It turns out that the same can be said for sleeping too much, too little, or getting just the right amount of shut-eye.

Scientists examined data on more than half a million U.K. residents ages 38 to 73 and discovered that both too much and too little sleep were associated with diminished cognitive performance. Long and short sleepers had trouble with processing speed, visual attention, memory, and problem-solving, as well an increased risk for depression.

The study, published in the journal Nature Aging, found that both genetics and structural changes in the brain (smaller brain volume/thickness) can contribute to the problems.

Those getting too little shut-eye may experience disruption of their deep sleep cycle, and that interferes with memory consolidation, promotes proliferation of amyloid tangles associated with Alzheimer's disease, and hampers the brain's ability to rid itself of toxins.

People who stay in bed too long may do so because of poor-quality and fragmented sleep that also disrupts their nightly deep sleep cycle.

What is just right? Seven hours a night, night after night, optimizes cognitive performance and mental health, say the researchers.

For help establishing a healthy sleep schedule, download the Cleveland Clinic's iOS app, Go To Sleep! or look for a behavioral sleep medicine program near you.

And make sure to get at least 30 to 60 minutes of physical activity daily. Plus, keep your bedroom dark, quiet, and cool.