How your body's clock affects your memory and thinking
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.
While performance on these tasks didn't change with the seasons, the amount of effort needed to accomplish them did. Brain activity involved in sustained attention (especially in the thalamus, amygdala and hippocampus) was highest in the summer and lowest in the winter. Brain activity associated with working memory (especially the pulvinar, insula, prefrontal and frontopolar regions), was higher in the fall and lower in the spring.
Seasonality, therefore, could be one factor in cognitive differences that occur for an individual tested at different times.
The finding is consistent with previous research showing seasonal variation in the levels and concentrations of certain compounds associated with mood (including dopamine and serotonin).
Participants were healthy young adults; it would be interesting to see if the same results are found in older adults. It's possible that the effects are greater.
 Meyer, C., Muto V., Jaspar M., Kussé C., Lambot E., Chellappa S. L., et al.
(2016). Seasonality in human cognitive brain responses.
Proceedings of the National Academy of Sciences. 201518129.
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.
A study involving genetically engineered fruit flies adds to our understanding of why sleep and bioclock disruptions are common in those with Alzheimer's disease. People with Alzheimer's often have poor biological rhythms — periods of sleep become shorter and more fragmented, resulting in periods of wakefulness at night and snoozing during the day. It has been thought that Alzheimer’s destroys the biological clock, but this new study indicates that the clock is still working — however, it’s being ignored by other parts of the brain.
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.
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. The circadian clock controls the daily cycle of sleep and wakefulness by inhibiting different parts of the brain by releasing GABA. It seems that if it’s not working right, if the hippocampus is overly inhibited by too much GABA, then the circuits responsible for memory storage don't function properly. The effect could be fixed by giving a GABA antagonist, which blocks GABA from binding to synapses. Recent mouse studies have also demonstrated that mice with symptoms of Down syndrome and Alzheimer's also show improved learning and memory when given the same GABA antagonist. The findings may also have implications for general age-related cognitive decline, because age brings about a degradation in the circadian system. It’s also worth noting that the hamsters' circadian systems were put out of commission by manipulating the hamsters' exposure to light, in a technique that was compared to "sending them west three time zones." The effect was independent of sleep duration.
Ruby, N.F. et al 2008. Hippocampal-dependent learning requires a functional circadian system. Proceedings of the National Academy of Sciences, 105 (40), 15593-15598.
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).
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.
Pyter, L.M., Reader, B,F. & Nelson, R.J. 2005. Short Photoperiods Impair Spatial Learning and Alter Hippocampal Dendritic Morphology in Adult Male White-Footed Mice (Peromyscus leucopus). Journal of Neuroscience, 25, 4521-4526.
Repeated, frequent episodes of jet lag without sufficient recovery time may reduce cognitive function
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.
Cho, K. 2001. Chronic 'jet lag' produces temporal lobe atrophy and spatial cognitive deficits. Nature Neuroscience, 4 (6), 567-568.
A hamster study indicates that chronic jet lag changes the brain in ways that cause long-lasting memory and learning problems.
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.
The findings support earlier research indicating that chronic jet lag impairs memory and learning and reduces the size of the temporal lobe, and points to the loss of brain tissue as being due to reduced neurogenesis in the hippocampus. Although further research is needed to clarify this, indications are that the problem is not so much fewer neurons being created, but fewer new cells maturing into working cells, or perhaps new cells dying prematurely.
Hamsters are excellent subjects for circadian rhythm research because their rhythms are so precise.
 Gibson, E. M., Wang C., Tjho S., Khattar N., & Kriegsfeld L. J.
(2010). Experimental ‘Jet Lag’ Inhibits Adult Neurogenesis and Produces Long-Term Cognitive Deficits in Female Hamsters.
PLoS ONE. 5(12), e15267 - e15267.
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