Born in Aldershot in 1959, Russell Foster is a professor of circadian neuroscience at Oxford and the director of the Nuffield Laboratory of Ophthalmology. For his discovery of non-rod, non-cone ocular photoreceptors he received numerous awards including the Zoological Society scientific medal. His latest book – the first he has written without a co-author – is Life Time: The New Science of the Body Clock, and How It Can Revolutionize Your Sleep and Health. What is circadian neuroscience? It’s the fundamental understanding of how our biology ticks on a 24-hour basis. But also it’s bigger than that – it’s an understanding of how different structures interact within the brain and how different genes and their protein products generate a complex behaviour. And that is then embedded throughout our entire biology. Is it an exciting field? What’s happened over the past 25 years has been a move into understanding how these internal 24-hour oscillations are generated and I think it’s one of the amazing success stories in biomedicine. One of the great aims of neuroscience is identifying different bits of the brain with different functions and here we’ve got one: the suprachiasmatic nucleus (SCN), with 50,000 cells, is the master circadian pacemaker. If you don’t have that, then all of our 24-hour rhythms just disappear. How did you first get interested in circadian research? It was largely through photoreceptors. During my second year as an undergraduate – I did zoology at Bristol – I was reading the extraordinary The Life of Vertebrates by JZ Young and I came across a bit about lampreys. They have a parietal third eye, which mammals don’t have; we only have ocular photoreceptors, whereas fish, reptiles, birds, all have multiple photoreceptors. And I just thought: wow, this is so cool. For my PhD, I was trying to understand how light is detected and measured to regulate the seasonal biology of birds. Then I started to address what seemed a simple question: how are the clocks of mammals regulated? We don’t have weird photoreceptors, we have visual cells that grab light in a fraction of a second and then forget it. So how can that light sensory system also be used to gather light information over long periods of time – dawn-dusk detectors? Way back in the early 1990s, we suggested that there was [an undiscovered photoreceptor] in the eye and there was a huge outcry. It wasn’t well received among your peers. No! But we just kept on kicking the door until they paid some attention. I think the really lucky thing for me has been doing zoology, because it’s looking at different evolutionary responses to similar problems, so you can draw parallels. Since I knew that there were weird photoreceptors in birds and fish, making the conceptual leap to saying: “Maybe there’s another photoreceptor within the mammalian eye” wasn’t so big for me. What prompted you to write this book? These extraordinary developments [in circadian and sleep research] are important to health across the spectrum, so part of the logic was: let’s try to make this as accessible as possible and help people to make informed decisions. Another strong element was a kick back against the sergeant majors of sleep screaming: “You must get eight hours” and: “You can’t look at a Kindle before you go to bed”, because it’s just wrong. Sleep is like shoe size: one size does not fit all. It’s an amazingly dynamic element of our biology. In public talks before lockdown, I remember a chap came up to me and said: “I don’t get eight hours of sleep; am I going to die?” And I said: “Well, yeah, you’re going to die, but not necessarily because you don’t get eight hours of sleep.” There’s a greater realisation of the importance of sleep, which is great, but in the process we’ve generated a lot of anxiety, so I want to take some of that anxiety away. The modern world has wreaked havoc on our body clocks. Has it been all for the bad? Being able to invade the night and do whatever you want because of cheap electric light has had a big impact upon the development of human societies, social interactions and behaviour. It’s also been quite democratising, but as always with humans, we take things a bit too far. We think we’re unfettered by our biology and of course we’re not. With this 24-hour biology, you can nudge it a bit one way or the other, but you do need to respect it or you run into problems. What are some of those problems? Night-shift work, for example, has a massive baggage of problems associated with it – 97% of shift workers do not adapt. And what are the consequences of that? High levels of coronary heart disease, cancer, immune suppression, metabolic syndrome, type 2 diabetes, obesity – all of those things are more prevalent, because we’re pushing our biology outside its normal range. That isn’t to say we’ve got to put the 24/7 society back in its bottle. It’s here to stay, but we’ve got to be aware of ways of mitigating it and employers, I think, should develop much more of a duty-of-care attitude. Are there such things as larks and owls, from a biological point of view? Absolutely. There’s a genetic predisposition to subtle changes in some of these clock genes [that] will speed the clock or slow it down. But it’s more than that. It changes through development and so from age 10 there’s a tendency to want to go to bed later and later. Women peak earliest, at 19-and-a-half, men peak at 21, and men tend to get later than women. As we grow older, we tend to become more morning again. So in our late 50s and early 60s, we’re getting up and going to bed at about the same times as when we were 10. It’s also to do with when you see light. [Seeing] dusk light delays the clock, making you get up later, while morning light advances the clock. All these things define whether you’re a morning type or not – and that’s really powerful information, because if you’re struggling, you can go outside [in the morning] or sit in front of a light box and that will help advance the clock. You won’t be able to negate it completely, particularly if you’re an owl, as I was, but you can at least shift in the right direction. So getting morning light into your eyes is a really important thing to do. Yes, for about 90% of us. But if you’re a real lark, then you do the opposite and make sure you’re getting evening light. Does it matter what time of day we exercise? There was a nice study a few years ago comparing athletic performance in early, mid and late types, and as you might predict the morning types’ athletic performance peaked at noon, the intermediate types at 3pm or 4pm and the late types at 7pm. So if you’re only going for power and peak performance, then it will matter and it correlates nicely with your chronotype. But it depends on what you want to do. If you really want to burn calories, then exercise before breakfast. If you want to go for peak athletic performance and be more energetic for longer, then do it later in the day. But be careful about not exercising too close to bedtime. That could raise core body temperature and part of sliding into sleep is a very slight loss in core body temperature. Is the timing of our meals important? And is dinner in the evening a bad idea? Terrible idea. Maimonides said: “Eat like a king in the morning, a prince at noon and a peasant at dinner” and he got it right. Studies have shown that, essentially, our metabolism is geared up to take in and burn calories during the first part of the day and lay them down as fat during the later part of the day; if you’re not exercising late, you’re not going to burn those calories up. In the study, people lost much more weight on an earlier eating schedule. We tend to think of those great Tudor banquets at night, but actually it was lunchtime. It was only with the aristocrats saying: “We’re liberated from the loss of light” that eating later became a thing. It was almost like a display of wealth and then eventually it spread across all of society. Tell me about chronopharmacology. This is so important. I think the future of medicine is going to be very much influenced by this, as well over 100 drugs have different time-of-day effects. There was a paper in 1993, on kids with a really severe form of leukaemia, and they either had morning or early evening treatment. And over the study period, 70% were still alive on the evening treatment, whereas it was 35% on the morning treatment. Ovarian cancer, same thing: over a five-year period, a study showed survival was around 45% on one time of day, 10% on another. You write that decision-making skills can vary depending on the time of day. Yeah, that’s interesting. A study from Australia compared the level of cognitive impairment with alcohol versus cognitive impairment at 4am. The level of impairment with time of day was actually greater. So if you’re driving your car in the early hours of the morning, your ability to drive is worse in terms of cognition than if you were legally drunk. I think this is really interesting, but what has our education system done? Well, we put all the so-called demanding subjects – maths, science and so on – first thing in the morning. And of course the kids are asleep during these critical morning windows and then bouncing off the walls in the afternoon. On the basis of the data, we might think it sensible to move some of those demanding classes to the afternoon and put sport in the morning. Is there an optimal time of day to conceive? It’s difficult [to say] from the female side, but from the male side, sperm quality is way better first thing in the morning than later on in the day – some Japanese studies have shown this. I got the most incredible email two nights ago from somebody who heard me on the radio. He said: “My wife and I have been trying to conceive a child for the past two years. We thought we’d tried everything but we’d forgotten about circadian rhythms. We switched to doing it in the morning and it worked first time.” Well, that’s an awesome responsibility! What are three simple things we can all do to improve our sleep? The overarching thing is that stress and anxiety are the enemies of sleep, so at the end of the day you’ve got to find ways of de-stressing. As a mechanistic cellular biologist, I always thought mindfulness is a bit like crystal-waving. But it’s not. The evidence is very clear that it really helps you take possession and de-stress. We’ve talked about getting morning light to set the clock. It also helps really making the bedroom a haven for sleep. Of course, it depends on one’s individual economic circumstances, because the bedroom is often the office, particularly during lockdown, but ideally, get rid of all that stuff that will distract you, make sure that the bedroom is dark and cool and invest in a decent mattress and pillows. Has writing this book prompted you to change your own lifestyle? It’s helped me define when I work best. Even though I’m old, I am still a late type and one of the great glories of becoming a full professor is that you decide what time the meetings start. Eating times has been a big thing. And exercise: I do a short 10-minute session before breakfast and then do a more sustained bout of exercise later in the day. Life Time: The New Science of the Body Clock, and How It Can Revolutionize Your Sleep and Health by Russell Foster is published by Penguin (£16.99). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply