Two experiments involving a total of 191 volunteers have investigated the parameters of sleep’s effect on learning. In the first experiment, people learned 40 pairs of words, while in the second experiment, subjects played a card game matching pictures of animals and objects, and also practiced sequences of finger taps. In both groups, half the volunteers were told immediately following the tasks that they would be tested in 10 hours. Some of the participants slept during this time.

As expected, those that slept performed better on the tests (all of them: word recall, visuospatial, and procedural motor memory), but the really interesting bit is that it turned out it was only the people who slept who also knew a test was coming that had improved memory recall. These people showed greater brain activity during deep or "slow wave" sleep, and for these people only, the greater the activity during slow-wave sleep, the better their recall.

Those who didn’t sleep, however, were unaffected by whether they knew there would be a test or not.

Of course, this doesn’t mean you never remember things you don’t intend or want to remember! There is more than one process going on in the encoding and storing of our memories. However, it does confirm the importance of intention, and cast light perhaps on some of your learning failures.

Another study has come out proclaiming the cognitive benefits of walking for older adults. Previously sedentary adults aged 55-80 who walked around a track for 40 minutes on three days a week for a year increased the size of their hippocampus, as well as their level of BDNF. Those assigned to a stretching routine showed no such growth. There were 120 participants in the study.

The growth of around 2% contrasts with the average loss of 1.4% hippocampal tissue in the stretching group — an amount of atrophy considered “normal” with age. Although both groups improved their performance on a computerized spatial memory test, the walkers improved more.

The findings are consistent with a number of animal studies showing aerobic exercise increases neurogenesis and BDNF in the hippocampus, and human studies pointing to a lower risk of cognitive decline and dementia in those who walk regularly.

A study involving 80 college students (34 men and 46 women) between the ages of 18 and 40, has found that those given a caffeinated energy drink reported feeling more stimulated and less tired than those given a decaffeinated soda or no drink. However, although reaction times were faster for those consuming caffeine than those given a placebo drink or no drink, reaction times slowed for increasing doses of caffeine, suggesting that smaller amounts of caffeine are more effective.

The three caffeine groups were given caffeine levels of either 1.8 ml/kg, 3.6 ml/kg or 5.4 ml/kg. The computerized "go/no-go" test which tested their reaction times was given half an hour after consuming the drinks.

In another study, 52 children aged 12-17 drank flattened Sprite containing caffeine at four concentrations: 0, 50 mg, 100 mg or 200 mg. Changes in blood pressure and heart rate were then checked every 10 minutes for one hour, at which point they were given a questionnaire and an opportunity to eat all they wanted of certain types of junk food.

Interestingly, there were significant gender differences, with boys drinking high-caffeine Sprite showing greater increases in diastolic blood pressure (the lower number) than boys drinking the low-caffeine Sprite, but girls being unaffected. Boys were also more inclined to report consuming caffeine for energy or “the rush”, than girls were.

Those participants who ingested the most caffeine also ate more high-sugar snack foods in the laboratory, and reported higher protein and fat consumption outside the lab.

Following on from previous studies showing that drinking beet juice can lower blood pressure, a study involving 14 older adults (average age 75) has found that after two days of eating a high-nitrate breakfast, which included 16 ounces of beet juice, blood flow to the white matter of the frontal lobes (especially between the dorsolateral prefrontal cortex and anterior cingulate cortex) had increased. This area is critical for executive functioning.

Poor blood flow in the brain is thought to be a factor in age-related cognitive decline and dementia.

High concentrations of nitrates are found in beets, as well as in celery, cabbage and other leafy green vegetables like spinach and some lettuce. When you eat high-nitrate foods, good bacteria in the mouth turn nitrate into nitrite. Research has found that nitrites can help open up the blood vessels in the body, increasing blood flow and oxygen specifically to places that are lacking oxygen.

Do retired people tend to perform more poorly on cognitive tests than working people because you’re more likely to retire if your mental skills are starting to decline, or because retirement dulls the brain?

For nearly 20 years the United States has surveyed more than 22,000 Americans over age 50 every two years, and administered memory tests. A similar survey has also been taking place in Europe. A comparison of the 2004 data for the U.S., England, and eleven European countries (Austria, Belgium, Denmark, France, Germany, Greece, Italy, The Netherlands, Spain, Sweden, and Switzerland) has now revealed differences in the level of cognitive performance among older adults between the countries (the 60-64 year age group was used as it represents the greatest retirement-age difference between nations).

These differences show some correlation with differences in the age of retirement. Moreover, the differences also correlate to differences in government policy in terms of pensions — supporting the view that it is retirement that is causing the mental decline, not the decline that brings about early retirement.

Memory was tested through a simple word recall task — recalling a list of 10 nouns immediately and 10 minutes later. People in the United States did best, with an average score of 11 out of a possible 20. Those in England were very close behind, and Denmark and Sweden were both around 10. Switzerland, Germany and the Netherlands, and Austria were all clustered between 9 and 9 ½; Belgium and Greece a little lower. France averaged 8; Italy 7; Spain (the lowest) just over 6.

Now when the average cognitive score is mapped against the percentage of retired for 60-64 year olds, the points for each country (with one exception) cluster around a line with a slope of -5, indicating that there is a systematic relationship between these two variables, and that on average being retired is associated with a lower memory score of about 5 points on a 20-point scale. This is a very large effect.

But the correlation is not (unsurprisingly) exact. Although the top scorers, U.S., England and Denmark, are among those nations who have lower retirement rates at this age, Switzerland has the same levels as the U.S., and Sweden has the fewest retired of all (around 40% compared to around 47% for the U.S. and Switzerland). Most interesting of all, why does Spain, which has around 74% retired, show such a low cognitive score, when five other countries have even higher rates of retirement (Austria has over 90% retired)?

There are of course many other differences between the countries. One obvious one to look at would be the degree to which older people who are not working for pay are involved in voluntary work. There’s also the question of the extent to which different countries might have different occupation profiles, assuming that some occupations are more mentally stimulating than others, and the degree to which retired people are engaged in other activities, such as hobbies and clubs.

The paper also raises an important point, namely, that retirement may be preceded by years of ‘winding-down’, during which workers become progressively more reluctant to keep up with changes in their field, and employers become increasingly reluctant to invest in their training.