A survey of 824 undergraduate students has found that those who were evening types had lower average grades than those who were morning types.
The finding was presented at SLEEP 2008, the 22nd Annual Meeting of the Associated Professional Sleep Societies (APSS).
Twice a week for four weeks, female hamsters were subjected to six-hour time shifts equivalent to a New York-to-Paris airplane flight. Cognitive tests taken during the last two weeks of jet lag and a month after recovery from it revealed difficulty learning simple tasks that control hamsters achieved easily. Furthermore, the jet-lagged hamsters had only half the number of new neurons in the hippocampus that the control hamsters had.
It’s not just a matter of quantity; quality of sleep matters too. A study involving 72 adults (average age 40), whose sleep was monitored for 11 consecutive nights, has revealed that reaction times on a morning psychomotor vigilance task was significantly slower after exposure to recorded traffic noise during sleep. The slowing was directly related to the frequency and sound-pressure level of the nightly noise.
We know circadian rhythm affects learning and memory in that we find it easier to learn at certain times of day than others, but now a study involving Siberian hamsters has revealed that having a functioning circadian system is in itself critical to being able to remember. The finding has implications for disorders such as Down syndrome and Alzheimer's disease. The critical factor appears to be the amount of the neurotransmitter GABA, which acts to inhibit brain activity.
A study involving adult male white-footed mice may help us understand seasonal dysfunctions such as seasonal affective disorder. The study found that those mice kept in artificial light conditions mimicking winter (8 hours of light per day) had impaired spatial memory compared to mice kept in “summer” conditions (16 hours per day). They also had, on average, smaller brains, with a proportionally smaller hippocampus, as well as changes in dendritic spine density in that region. Other types of memory did not appear to be affected.
A study of 20 flight attendants suggests that people who undergo repeated, frequent episodes of jet lag without sufficient recovery time between trips may develop actual tissue changes in the brain in an area that's involved in spatial orientation and related aspects of cognitive function. The extent to which this is due to sleep deprivation rather than time shifts per se is unknown. These findings may also be relevant to shift workers, medical trainees and others who work long hours.
A study involving mice lacking a master clock gene called Bmal1 has found that as the mice aged, their brains showed patterns of damage similar to those seen in Alzheimer's disease and other neurodegenerative disorders. Many of the injuries seemed to be caused by free radicals. Several key antioxidant enzymes, which usually neutralize and help clear free radicals from the brain, have been found to peak in the middle of the day in healthy mice, but not in these mice lacking Bmal1.
A sleep study involving 28 participants had them follow a controlled sleep/wake schedule for three weeks before staying in a sleep laboratory for 4.5 days, during which time they experienced a cycle of sleep deprivation and recovery in the absence of seasonal cues such as natural light, time information and social interaction. The same participants went through this entire procedure several times over some 18 months. Brain activity was assessed while participants undertook an n-back working memory task, and a task that tested sustained attention.
Harrison, Yvonne & Horne, James A. 2000. Sleep loss and temporal memory. The Quarterly Journal of Experimental Psychology, 53A (1), 271-279.
Recognition memory for faces was unaffected by being deprived of sleep for 35 hours.
However, sleep-deprived subjects were significantly worse in remembering in which of two sets of photos particular faces had appeared in.
Sleep-deprived subjects who had been given significant doses of caffeine remembered the set better than those who had not, but were still poorer at remembering than those not deprived of sleep.
Although their performance was poorer, sleep-deprivation seemed to increase the subjects' belief in their own accuracy.
It seems likely that sleep-deprivation affects memory for context.
In this study, subjects were shown two sets of 12 color photographs of people’s faces (24 in total). Five minutes after seeing the last one, the subjects were then shown another 48 faces (one by one, as before) and had to say whether or not they had seen the face earlier. If so, they were asked whether they saw it in the first or second set of photographs. Half the subjects had been deprived of sleep for the previous 35 hours. Some of these had been given significant amounts of caffeine to offset their sleepiness.
It was found that the sleep-deprived subjects, whether or not they had had caffeine, were as good as the non-sleep-deprived subjects at recognizing which faces they had seen before. However, the sleep-deprived subjects were significantly worse at remembering which set the faces had appeared in. This occurred even though otherwise optimum conditions for recall existed (the test was novel, stimulating, and relatively short; it was given at the best time of day for maximum alertness).
Caffeine significantly reduced the feelings of sleepiness and did appear to improve the ability of the sleep-deprived subjects to remember which set the face had appeared in, but the level of recall was still significantly below the level of the non-sleep-deprived subjects. Caffeine made no difference to the memory performance of subjects who were not sleep-deprived.
Interestingly, sleep deprivation increased the subjects’ belief that they were right, especially when they were wrong. In this case, whether or not they had had caffeine made no difference.
It may be that the problem with temporal memory reflects a more general problem with remembering context information.
A mouse study suggests that merely changing meal times could have a significant effect on the levels of triglycerides in the liver. Levels of triglycerides followed a circadian rhythm, with levels peaking about eight hours after sunrise (note that mice are nocturnal). Mice generally eat 20% of their food during the day, and 80% at night. Mice lacking a functional body clock eat constantly during the day. When normal mice were given the same amount of food, but had to eat it only at night, there was a quick and dramatic 50% decrease in overall liver TAG levels.
Folkard, S. & Monk, T.H. (1978). Time of day effects in immediate and delayed memory. In M. M. Gruneberg, P. E. Morris & R.N. Sykes (eds.). Practical aspects of memory. London: Academic Press.
In other words, what's important is the time of day you hear/see/read something, not when you try and remember it.
Three experiments investigated whether the time of day had an effect on short-term or long-term memory.
In the first experiment, the material used was a factual article from a New Scientist magazine. Short-term memory (as measured by 10 multi-choice questions) was best if the article had been read at 8am, and lowest if it had been read at 8pm. Surprisingly, there was a slight, short-lived improvement after lunch (during the post-lunch dip in arousal level), and another one after 8pm (at a similar dip in arousal). Long-term memory (as measured by performance on a category instance task) was apparently not affected by time of day. Nor was reading speed.
In the second experiment, the subjects listened to a story, at either 9am or 3pm. Their recall was tested immediately and again a week later. It was found that short-term recall was better if the story was heard at 9am but long-term recall was better if it had been heard at 3pm. It didn't seem to matter whether testing occurred at 9am or 3pm, nor did it matter whether the test occurred at the same time of day as the story was heard.
In the third experiment, the subjects were shift workers. The subjects, who were nurses, were shown a ten minute film on the use of radium therapy. The times used were more extreme this time: 8.30pm and 4am. Long-term recall was tested at four weeks. Long-term recall was consistently better if the film had been seen at 8.30pm than if it had been seen at 4am, but there was no effect on immediate recall. However, in the group least adjusted to shift work (part-time nurses and those on their first night shift), short-term recall was better if the film had been seen at 4am, while among the most adjusted group the reverse was true (short-term recall was best if the film had been seen at 8.30pm). Again, the time of testing made no difference.
Overall then, the experiments found that the time at which the information was presented consistently influenced immediate and delayed retention in opposite directions. It is not clear why there should be a differential effect. There was no evidence that retrieval efficiency was affected by the time of day.
An interesting implication of this work is that the recommendation from studies early this century, that academic work is better taught in the morning and physical subjects in the afternoon (based on findings that immediate memory was better in mid-morning, and perceptual-motor activity in the afternoon), may have been ill-founded.