28 October 2016
How your “body clock” may affect cognition
Is it true that "early birds" actually do have better concentration in the morning and can cognitive tests show patterns or changes over time, even within a few hours?
Most people experience variations in their level of attention day to day, and in everyday life people frequently refer to it: “I’m not fully awake yet” or “I had a really productive morning but I’ve lost concentration”, but how much of these differences from dawn to dusk can affect our cognitive functions. Is it true that “early birds” actually do have better concentration in the morning and can cognitive tests show patterns or changes over time, even within a few hours?
There are definite cyclical peaks in biological parameters such as temperature, blood pressure and the levels of certain hormones which occur on an approximately 24-h cycle as the body’s internal clock or circadian rhythm couples with external synchronizing factors, known as zeitgeber. The most well-known zeitgeber is light, but other cues in the environment such as meals, exercise and social cues also help regulate our circadian clock. It is important to note that every person has their own circadian preference or chronotype, and certainly not everyone would describe themselves as an early bird. As well as differences between people, there are trends across the lifespan. Older adults tend to be more “early birds” than the younger generation. However, because of these differences between individuals, it can be difficult to observe general trends across populations when their chronotype is not accounted for.
In addition to these physiological changes, there has been a long-standing recognition that our cognitive function also shows similar daily cycles. There has been intriguing research looking at the real world impact of time of day and fatigue on decision making. In one study, the decisions of parole-board judges were found to depend on when the case was heard, even taking into account the severity of the crime. The proportion of favourable rulings dropped from about 65% at the start of the day, to nearly zero. Interestingly, the rate of favourable judgements returned to 65% after the judge took a break.
In a controlled laboratory setting, there is also evidence that higher-level cognitive functions, related to planning and behavioural control are also affected by time of day. For instance, Manly (2002) showed a higher accuracy in the early afternoon and evenings on a go/no-go task compared to late at night and early in the morning. Interestingly, there is evidence that these diurnal changes in cognitive performance interact with factors such as age, which affect how much of a morning or evening person you are. For instance, a study by May et al. (1993) found that younger adults show a better recognition accuracy in the afternoon compared to mornings, but older adults showed the opposite effect.
Disruption of circadian rhythm
There has been considerable interest in what happens when circadian rhythms are disrupted, either through interventions, for instance sleep deprivation or by a disease process. Subjectively, lack of sleep is associated with the feeling of not being mentally sharp. Indeed, a study by Waller et al. (2015) found that in middle aged men, increased difficulty falling asleep is correlated with poorer performance across a wide range of CANTAB tests of memory, executive function and processing speed. When comparing the impact of sleep deprivation to doses of alcohol, 18–20 h of sleep deprivation yields performance decrements similar to that of a person who is legally intoxicated.
There are also many medical conditions which affect sleep, either by affecting the body or by directly impacting the brain. For instance, children with idiopathic juvenile arthritis may suffer from pain which interferes with normal sleep. Parents with clinically-significant sleep disruption were found to have worse performance on measures of reaction time (Ward 2011). Similarly, mood disorders affect 5-10% of the population and in the World Health Organisation Burden of Disease study depression was second in terms of associated disability. In addition to low mood, patients with depression frequently report cognitive slowing or difficulties with concentrating. There is even some evidence that depression itself impacts the normal circadian rhythms, with changes to the normal circadian patterns of hormonal release. The rise of cortisol in the morning is steeper in patients with depression than in unaffected individuals. During a depressive episode, mood symptoms often vary during the course of a day, and disruptions to sleep and eating are also present. These can be either a difficulty in sleeping and remaining asleep, or an excess of sleep, which in turn, research has shown can affect your cognitive functioning. We know that depression is accompanied by objectively measured impairments in memory, executive function and attention (Rock et al., 2014). These are now being targeted by pharmacological interventions for the first time. Given the disruptions to circadian rhythms seen in depression, Moffoot et al. (1994) looked at the impact of time of day on cognitive performance in patients with depression and healthy controls. They found that patients with depression performed more poorly than controls in the CANTAB Delayed Matching to Sample (DMS) task. These patients showed significantly worse performance in the afternoon than in the morning, a pattern which was not present in the healthy controls.
So what does this mean?
We know that normal circadian rhythms are associated with performance on certain cognitive tasks, particularly those requiring rapid information processing. We also know that disruption to these circadian rhythms through sleep deprivation, physical illness or depression results in decreases in performance the following day, but in some cases, can result in better performance at certain times of day. When using cognitive assessments in clinical trials, variation in measurements from time of day should be accounted for when developing your trial design. CANTAB cognitive assessments are sensitive to small variations in performance and can be used for assessing changes over time, whether that is 24 hours or 12 months. CANTAB has been used in studies assessing cognition in depression, sleep and over 100 other diseases and disorders. Contact us to find out more about using CANTAB in your study, or to discuss your research study design with our experts.
Colquhoun P (1981). Rhythms in performance. In: J. Aschoff (Ed.), Handbook of Behavioral Neurobiology (Vol. 4, pp. 333-347): New York: Plenum Press.
Kleitman N, Titelbaum S, Feiveson P (1938). The effect of body temperature on reaction time. American Journal of Physiology;121:495-501.
May CP, Hasher L, Stoltzfus ER (1993). Optimal time of day and the magnitude of age differences in memory. Psychological Science;4:326-330.
Moffoot APR et al. (1994). Diurnal variation of mood and neuropsychological function in major depression with melancholia. Journal of Affective Disorders;32(4):257-269.