In this episode, we talk with Professor Chiara Cirelli from the department of Psychiatry at the University of Wisconsin.

We begin by hearing about how she first got interested in sleep research, and particularly about her time as a medical student at the University of Pisa where she worked on the noradrenergic system using cat models. We then look at the importance of using cross-species models in sleep research with flies, mice, rats and humans. We also discuss the different ways of using these animal models and the different advantages they offer for sleep research.

We then move on to Chiara's Synaptic Homeostasis Hypothesis. This hypothesis has been supported by data from a wide range of species, and suggests that an important function of sleep is to downscale synapses. During the day, brain connections (or synapses) relating to information we have learned get strengthened. This can result in a kind of neural saturation, whereby there is no space for more synapses. The Synaptic Homeostasis Hypothesis proposes that these synapses are downscaled during sleep, bringing about a form of homeostasis that allows us to repeatedly strengthen synapses during wake and downscale during sleep.

We also take a look at different ways of measuring synaptic growth across species such as molecular studies, structural or electrophysiological studies and the current difficulties in following synapses over long periods of time.

Finally, we look at Chiara's most recent finding which shows synapses associated with new learning are protected from downscaling during sleep. Instead, her latest data show that it is the background noise that is reduced during sleep, allowing for an increase in signal to noise ratio.

We hope you enjoy the episode and please find more information below.

Here are links to some of the studies mentioned in the podcast:

• Synaptic Homeostasis Hypothesis
• Synaptic Homeostasis Hypothesis in Memory Consolidation
• Downscaling

If you would like to find more information of Chiara's work, you can find a link to her research page here.

Glossary of terms
Synaptic Homeostasis - Renormalistion of overall synaptic strength to restore cellular homeostasis, preventing saturation and allowing further memory formation following a day of learning

ATP 'cost' - ATP is our source of energy for everything that we do. An ATP 'cost' would refer to how much cellular energy something would use.

Declarative learning - Learning about knowledge which we can talk about e.g dates, facts, events.

Drosophila - a genus of flies, commonly referred to as fruit flies, which are used regularly in scientific research.

Smart down selection - A process of selectively protecting synapses which need to be kept during downscaling.

Phosphorylation - Process of adding a phosphoryl group to a molecule. Phosphorylation can help regulate cell signals or protein development often acting as a way of 'labeling' cells.

Noradrenergic System - A neurotransmitter system within our nervous system that is often associated with alertness or arousal.

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Episode produced by Sophie Smith

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