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Professor Russell Foster has written a new book about circadian neuroscience which is published by Penguin this week. This book review by Jacqueline Pumphrey was first published on the University of Oxford website.

Life Time is published by Penguin on 19 May 2022.

In his 1953 poem Days, Philip Larkin asks: 'What are days for? Where can we live but days?' He goes on with typical gloomy yet endearing angst: 'Ah, solving that question/Brings the priest and the doctor/In their long coats/Running over the fields.'

Larkin would have done well to include 'scientist' along with priest and doctor. More specifically, he might have been interested to talk to circadian neuroscientists, such as Professor Russell Foster, who has said that the commercialisation of electric light since the 1950s has allowed us to 'declare war upon the night' and think that we can 'do what we want, at whatever time we choose'.

Russell argues that in doing so, 'we have thrown away an essential part of our biology'. One recent example of this can be found in Samsung's 'Night Owl' television advertisement, which features a woman jogging through a city in the middle of the night (2.00am), tracking her progress with a Galaxy Watch. The advert encourages viewers to pursue their health and wellness goals on their own schedules – despite the overwhelming objective scientific evidence that this can be extremely harmful.

It is this insight about our arrogant 24/7 society that underpins Russell's new book, Life Time, published by Penguin on 19 May. Almost a handbook, Life Time presents the reader with a wealth of information on what scientists have discovered about sleep and biological circadian rhythms. The book is designed to help each of us 'make an informed and evidence-based decisions about improving our sleep and circadian health to improve our lives.' Life Time represents a generous sharing of a lifetime's work, and demonstrates Russell's commitment to the public understanding of science.

Russell Foster is perhaps best known for his team's contribution to the discovery in the late 1990s of photosensitive retinal ganglion cells, a type of neuron in the eye. Unlike the eye's rod and cone cells, they're not responsible for forming images, but for detecting light, providing information to the brain about the length of day and length of night. The ground-breaking discovery of these cells and their function was a milestone in the now exploding field of circadian biology to which Russell has dedicated his working life.

Photosensitive retinal ganglion cells are important because they help to ensure that the human body's circadian system, which is organised by a master 'body clock' in the brain, is kept in synchronisation with external light and dark. Without this mechanism, our rhythms would drift out of sync, leading to all sorts of problems. We are biological animals whose organs need to be ready during the active phase of the day to eat and process food, and during the resting phase to use stored energy to repair tissue, remove toxins, fight off infection, form memories, and generate new ideas.

Russell's interest in circadian rhythms is intertwined with his interest in the purpose and mechanisms of sleep, which led him to set up the Sir Jules Thorn Sleep and Circadian Neuroscience Institute at the University of Oxford in 2012. Scientists here are uncovering more about the fundamental ancient processes that govern time in our lives. They are investigating what happens when sleep and circadian rhythms are disrupted, for example in shift workers, or in those of us who think we can somehow live outside days, pushing our waking hours into the night. Being out of sync can lead to increased stress hormones, heart disease, weight abnormalities, reduced immunity, increased risk of cancer, and emotional and cognitive problems. No wonder Russell was motivated to share some of the cutting-edge insights from the scientific community in order to help people make good decisions about how to live healthy lives.

So what are the practical take-home messages of this book? Perhaps the most surprising one is that there is no 'one size fits all' in terms of sleep. It's not helpful to think that everyone should be aiming for eight hours sleep a night, for example, or even that the holy grail is to sleep right through the night without waking up. The sleep that you need is linked to your genetics and environment and can change over the course of your life. The book includes pointers for how to determine your own 'chronotype' – do you function best in the morning, the evening, or in between? Armed with this information you can adapt your behaviour to synchronise your internal clock to external time in the best way for you.

There are specific messages about the best time for various health interventions: stroke medications such as aspirin should be taken before you go to sleep rather than in the morning, because aspirin turns-off the "stickiness" of platelets which are made at night; a flu vaccine is better given in the morning because the immune system is upregulated during the daytime. Russell explains the scientific evidence for behaviours that will improve our sleep, such as not drinking caffeinated drinks after lunchtime, and not eating a large meal or discussing potentially difficult issues immediately before bedtime.

The advice in this book is not delivered in a didactic fashion by an aloof expert from the hallowed halls of academia, but by an engaging and entertaining human being who is clearly excited by knowledge and passionate about sharing it. Academics are used to writing, spending long hours documenting the methodology and results of their experiments, and writing applications to persuade funding bodies to provide grants to allow that research to continue. But only a few are motivated or skilled enough to write about the insights of their work for the benefit of ordinary people who want to live their lives to the full.

Russell has clearly enjoyed digging deep into the topic that has fascinated him for his whole life, and then distilling the findings and making them accessible and relevant. His vast experience of giving public talks has enabled him to get inside the minds of his audience, and he has used genuine questions from people attending such events to form the basis of the wide-ranging and sometimes quirky Q&A sections at the end of each chapter. This book demonstrates the most practical purpose of the scientific endeavour – to apply the lessons learned in order to improve our everyday lives.

To return to Philip Larkin and what days are for, let us leave the answer to Russell, whose hope for his book is that it can help us to 'be healthier, more creative, make better decisions, gain more from the company of others, and view the world and all that it has to offer with a greater sense of curiosity and wonder'.

Life Time is published by Penguin on 19 May 2022.