When people ask me „What is your favorite leisure activity?” the first thing that comes to my mind is: sleeping. Well, this will not be my answer in most of the cases as sleeping is interpreted as something lazy, unproductive, and for a conversation as a boring ‘activity’ and therefore not the best way to make a good impression on someone you do not know. But what if this view is false, if it is in fact a very useful way to spend your free time, because it makes you smarter?

It is well known amongst researchers that sleep is one crucial factor determining cognitive functioning. Performance in decision making, language, attention, working memory, and various other cognitive processes can be impaired by even small sleep deprivation. On the other hand sleep also has an enhancing potential regarding cognition. The main focus of research in cognitive enhancement during sleep is on memory, which is comprised in most other cognitive processes. Memories of past experiences help us to behave correctly in present events and furthermore create expectations of future situations.

In order to improve memory beyond the normal condition two approaches can be used. Firstly, the timing of sleep is important. Naps in advance to learning a new content result in an improved initial acquisition and encoding of freshly created memory traces. Furthermore, sleep subsequent to the process of encoding leads to a support in memory consolidation processes. Thus, memories are preserved for long-term storage and less susceptible of interfering inputs.
The cause of these effects may be that activity patterns of the actual learning experience are reactivated during sleep experience. These reactivations might support the underlying memory traces and enable them to be incorporated into memory networks associated with long-term storage.

The second approach of memory enhancement during sleep is the manipulation of sleep and/or memory. On the one hand these manipulations can intervene with the processing of particular memory representations (memory reactivation), for example by means of odor or acoustic cues. On the other hand certain sleep features which are related to memory processing, such as specific brain oscillations, can be manipulated. Manipulation can also be applied by pharmacological medications affecting characteristic neurotransmitters such as noradrenaline.

Both approaches, sleep itself and manipulation of memory, result in a decrease of forgetting and an increase in memory stability and therefore an overall enhancement of memory. However, procedural (resulting from one’s behavior) and declarative (verbally describable) learning while sleeping appears to be impossible. On the other hand new knowledge can be acquired during sleep by generalizing, integrating and abstracting schemata and converting implicit into explicit knowledge on the basis of previously encoded memories.
Studies suggest that natural sleep increases human memory by 10-20% compared to wakefulness. Furthermore, manipulation of memories may lead to an enhancement of 5-15% compared to normal sleep. Therefore, manipulation results in an improvement of 15-35% in memory compared to wake and not manipulated phases.

Even though the manipulations of sleep are only applicable to some extent, even usual sleeping before or after a learning task shows a clear advantage in memory, which makes it a very useful means of learning. Therefore, when now someone asks me what I like to do in my spare time I will proudly answer: “I like sleeping”.