I have two critical issues with this book:

a) the central circadian controller is light – the amount of information on light in this book is minimal at best

b) the focus of this book is 75% on the four chronotypes that Dr Breus created himself, for which almost no scientific references are cited (because they do not exist)

Circadian rhythms are species-wide, with minimal differences within a species. And when significant differences occur they go along with circadian disruptions and an eventual decline in health. What I saw recently exemplifies this nicely: picture a rat in the middle of the day on a sunny lawn. The poor rat was stumbling along in a haze, obviously very confused and ill. No surprise as rats are nocturnal creates. We are diurnal. Our biology evolved around the movement of the sun. This is controlled by the light signal we receive through our eye and our skin. The master clock in the brain is governed by light received through photoreceptors in the eye. When and what light you see sets your main rhythm. Everything else is a secondary input—for example, peripheral clocks in your gut, which are set by when you eat. 

My point is that you cannot really talk about circadian rhythms without discussing and understanding light in some depth. Indigenous people rise before/around sunrise and their sleep various according to the season [1]. Think longer days with shorter nights in summer and longer nights and shorter days in winter. Why? Because of the light and temperature changes. 

Night owls only really exist within modern societies that are bathed in artificial light. When researchers took a group of proclaimed night owls camping for seven days, without any artificial light source, their circadian rhythms moved closer to the movement of day and night as they naturally went to bed earlier and woke up earlier [2]. In other words, humans living under conditions of natural lighting physiologically adapt to the length of day [3].

This shift in sleep/wake patterns is not surprising as artificial light sources are high in blue light while red and infrared light is mostly absent. It is the blue light part of the light spectrum that the photoreceptors in your eye are most sensitive too. This is why artificial light (think LEDs, fluorescents, mobile phone, computer etc) is the leading circadian disruptor that messes with your hormones, sleep and health. [4]

Without reducing and blocking artificial light, especially at night, you will majorly disrupt your circadian rhythms. The other part of the equation is getting more natural light exposure during the day, especially in the morning. It is no secret that seeing the sunrise consistently is by far the most important thing you can do for your circadian rhythm integrity. When living by this natural rhythm, the difference in sleep/wake patterns between so-called night owls and early birds becomes minimal. The same goes for the coined chronotypes dolphins, lions, bears and wolves. Such a distinction is helpful if you do not change your light environment. In that case, you will see improvements following “your chronotype” schedule. But this schedule is far from optimal as your circadian rhythms will still be majorly disrupted unless you take the sun as your guide and learn about light.

When you start your day after sunrise, you miss out on natures wake up call and the hormonal boost of adrenaline and cortisol that the increasing amount of blue light at sunrise provides. 

In short: the book makes some good points on circadian rhythms in general but entirely misses crucial information on light and managing one’s light environment, all the while focussing on a distinction of four so-called chronotypes that have hardly any bearing in the scientific literature.

References:

[1] Yetish, Gandhi et al. “Natural sleep and its seasonal variations in three pre-industrial societies.” Current biology : CB vol. 25,21 (2015): 2862-2868. doi:10.1016/j.cub.2015.09.046

[2] Stothard, Ellen R et al. “Circadian Entrainment to the Natural Light-Dark Cycle across Seasons and the Weekend.” Current biology : CB vol. 27,4 (2017): 508-513. doi:10.1016/j.cub.2016.12.041

[3] Emens, Jonathan S. et al. “Circadian Rhythms: The Price of Electric Light”. Current Biology vol. 27,4 (February 20, 2017): PR144-R145. doi:10.1016/j.cub.2017.01.014

[4] Grubisic M, Haim A, Bhusal P, Dominoni DM, Gabriel KMA, Jechow A, Kupprat F, Lerner A, Marchant P, Riley W, Stebelova K, van Grunsven RHA, Zeman M, Zubidat AE, Hölker F. Light Pollution, Circadian Photoreception, and Melatonin in Vertebrates. Sustainability. 2019; 11(22):6400. doi:10.3390/su11226400