Sleep is a fascinating process that allows most of the creatures on Earth to function at their normal capacity and survive the environmental challenges. Recent theories support the idea that one of the main functions of sleep is not energy conservation or tissue restoration, as previously thought, but actually, enhancement of brain function. Sleep appears to be involved in consolidation of memories and improvement of learning. As you probably remember from the previous article, there are essentially to phases of sleep: REM and non-REM. The latter has been shown to be involved in consolidation of both declarative and non-declarative memories, whereas REM sleep is believed to enhance the formation of procedural and emotional (non-declarative) memories.
If you’ve ever wondered whether insects can sleep or not, the answer is yes. Several studies in Drosophila melanogaster , the fruit fly famous for Morgan’s discoveries of chromosomal inheritance, proved these flies have periods of ‘rest’. During rest , Drosophila stops moving and becomes more difficult to be aroused, which resembles what we understand by ‘sleep’. Moreover, it appears that these flies can form short-term and long-term memories, especially revealed by studies of olfactory associative conditioning. Surprising, isn’t it? Even though it is known that insects have brains, they seem too primitive, at a first glance, for complex cognitive functions such as memory and learning. Well, it’s obvious that insects are much smarter than we thought.
What’s more stunning is that, in Drosophila, just as in humans, sleep has evident effects on long term memory formation and sleep deprivation can dramatically affect this function. According to several studies, sleep homeostasis positively influences learning. During non-REM sleep, the brain shows slow oscillatory activity, which is characterised by waves of action potentials with low frequency and high amplitude. These slow waves are controlled by homeostatic processes and increase after learning tasks. Hence, sleep, which induces slow wave activity, plays a very important role in enhancing learning performances.
The homeostatic control of sleep has been somewhat elucidated by studies in our old friend, Drosophila. Organisms need not only a circadian clock (which sets a day-night rhythm, synchronising the body with the environment), but also a homeostatic system, which regulates sleep according to prior wakefulness. Just like in the circadian system, the homeostatic system seems to be genetically regulated and, eventually, resulting from neuronal activity.
Specialised sleep-promoter neurones in Drosophila, called the dorsal fan-shaped body (FB) neurones, become excited when the organism is sleep-deprived and fire action potentials (which can be surprising, given that non-REM sleep triggers reduced brain activity). It is not exactly known how the nervous system can sense lack of sleep, but it is believed that substances such as adenosine and unfolded proteins, released after prolonged wakefulness, are potential signals. Thus, after detecting too much wakefulness, dorsal FB neurones become excited and promote sleep. The gene that controls the function of the dorsal FB neurones is the crossveinless-c (cv-c) gene, which encodes for a protein that regulates different ion channels (especially potassium channels), leading to changes in electrical conductance and the excitability of the sleep-promoter neurones.
Just to give some more information and (possibly) complicate things a bit more, another link between sleep and long term memory is represented by the notch receptors, which have been found to be involved in the restoration of long term memory formation after sleep deprivation. Their main function is to influence the development of the nervous system in the embryo, but they also play a role in memory formation.
As you have probably figured out by now, it is quite hard to have a complete picture of what sleep is about and how it can influence memory and learning. However, scientists are doing their best to shed some light on these wonderful phenomena. And before you close this page, don’t forget an important point: even something small like the fruit fly is able to sleep and learn!
For further information:
Ackermann, S., Rasch, B., 2014. Differential effects of non-REM and REM on memory consolidation, Current neurology and neuroscience reports, vol. 14, p. 430
Donlea, J. M., Pimentel, D., Miesenbock, G., 2014. Neuronal machinery of sleep homeostasis in Drosophila. Neuron, vol. 81, pp. 860-872
Huber, R., Ghilardi, M. F., Massimini, M., Tononi, G., 2004. Local sleep and learning. Nature, vol. 430, pp. 78-81