Biological clocks and memory

I’ve always been interested in the body’s clocks — and one of the most interesting things is that it is clocks, in the plural. It appears the main clock is located in a part of the brain structure called the hypothalamus (a very important structure in the brain, although not one of much importance to learning and memory). The part of the hypothalamus that regulates time is called the suprachiasmatic nuclei. These cells contain genes that switch on, off, and on again over a 24-hour period, and send electrical pulses and hormones through the body. This is the body’s master clock.

But it is not the only clock in the body. Each organ in the body uses the time signal from the master clock to set its own clock. As a consequence, different systems in the body operate on different schedules. Thus blood pressure peaks at one particular time of the day, and levels of the stress hormone cortisol rise and fall in accordance with the clock that governs this.

The effect of this is that certain physical disorders are more likely to occur at particular times, and, more significantly, that certain medications may be far more effective at certain times.

What does all this have to do with learning and memory?

Well, not a whole lot of research has been done on the effects of time of day on cognitive performance, but what has been done is reasonably consistent. It seems clear that, for many people (but not all), there are significant time of day effects. The most reliable is that, in general, teenagers and young adults perform best (mentally) in the afternoon, while older adults (seniors) perform best in the morning.

Having said that, let’s qualify it a little.

Let’s start with a table. Now, this represents the findings of one study [4], so let’s not get carried away with the illusion of precision cast by actual numbers. Nevertheless, it is interesting. These percentages represent the preferences reported by the young and old participants in the study. These preferences correlated with improved performance on a memory test.

  Young Old
Definite morning 0% 34%
Moderate morning 8% 49%
No preference 57% 10%
Moderate evening 29% 6%
Definite evening 6% 1%

Now the first thing to note is how marked the differences are between young and old. Of particular interest is how many of the younger adults had no preference. Compare this with that of older adults. The second finding of particular note is how pronounced the preference for the morning is in older adults — 83% preferred morning. And, most interesting of all, is a finding from another study by the same researchers [5]: when tested at their preferred time, older adults performed comparably to younger adults on a memory task. Younger adults, by contrast, seem able to perform well at all times.

There is also some evidence [3] that the deleterious effect of interference (the intrusion of irrelevant words, objects, events) is worse for older adults at those times of day when their performance is poorer. Older adults are more vulnerable to interference than younger adults.

The findings for teenagers and young adults may also apply to children. One study [2] found that below-grade-level students who received reading instruction in the afternoon improved their performance more than those students who received instruction in the morning.

But it must always be remembered that this general principle that morning is better for the aged, and afternoon better for the young, does not apply to each and every individual. As the table tells us, time of day affects some people more than others, and time preference is an individual matter, not entirely predicted by age. This is underscored by a study [1] that found improved performance when students were taught at times that matched their preferences. There was also some evidence that, for some students at least, achievement was greater when they were taught during their teacher's ideal time of day.

None of this is an argument that you should resign yourself to learning only at your preferred time of day! But you could use the information to modify your strategies. For example, by scheduling difficult work for your optimal time (assuming you have an optimal time, and are not one of those fortunate people who have no strong preference). You can also try and counteract the effect by, for example, drinking coffee during your nonoptimal time of day (this was found to be effective in one study with older adults [6]).


  1. Ammons, T.L., Booker, J.L. & Killmon, C.P. 1995. The effects of time of day on student attention and achievement. (ERIC Document Reproduction Service No. ED 384 592)
  2. Barron, B., Henderson, M. & Spurgeon, R. 1994. Effects of time of day instruction on reading achievement of below grade readers. Reading Improvement, 31(1), 56–60.
  3. Hasher, L., Chung, C., May, C.P. & Foong, N. 2002. Age, Time of Testing, and Proactive Interference. Canadian Journal of Experimental Psychology, 56, 200-207.
  4. Intons-Peterson, M.J., Rocchi, P., West, T., McLellan, K. and Hackney, A. 1998. Aging, optimal testing times, and negative priming.Journal of Experimental Psychology: Learning, Memory, and Cognition, 24(2), 362-376.
  5. Intons-Peterson, M.J., Rocchi, P., West, T., McLellan, K. and Hackney, A. 1999. Age, testing at preferred or nonpreferred times (testing optimality), and false memory. Journal of Experimental Psychology: Learning, Memory, and Cognition, 25(1), 23-40.
  6. Ryan, L., Hatfield, C. & Hofstetter, M. 2002. Caffeine Reduces Time-of-Day Effects on Memory Performance in Older Adults. Psychological Science, 13 (1), 68-71.
  7. West, R., Murphy, K.J., Armilio, M.L., Craik, F.I.M. & Stuss, D.T. 2002. Effects of Time of Day on Age Differences in Working Memory. Journals of Gerontology Series B, 57 (1), P3-P10