They begin with an introduction to daily (circadian) rhythms in the natural world, a look at the abilities of animals such as bees to tell time, and some general background on clocks and oscillators. Organisms can better meet the challenge posed by daily changes if they can predict them.
The search for a clock led, through experimental work on rats, hamsters and other mammals, to the suprachiasmatic nuclei. Circadian clocks are kept in time by entrainment to the daily cycle, with a similar phase response cycle in different species; investigation of the details uncovered additional mammalian light sensors in addition to the well-known rods and cones.
Work on Drosophila has revealed the molecular workings of one clock, where per and tim genes produce proteins which provide negative feedback on their expression. Much remains poorly understood, however, and other species have different systems — there is considerable variation just among birds, involving different parts of the brain and different genes and proteins.
Seasonal or circannual rhythms are especially important in reproduction and migration; they are often controlled by critical day lengths, and are thus connected to circadian rhythms. Turning to evolution, attempts have been made to show experimentally that circadian rhythms are adaptive, while research into cyanobacteria, mammals, insects, and plants has revealed something of their phylogenetics.
The last third of Rhythms of Life turns to humans. Performance and physiology vary during the day, and sleeping patterns also interact with circadian rhythms. Modern phenomena such as shift-work and jet-lag can have drastic negative impacts on circadian rhythms and well-being. (A brief appendix provides some practical tips on alleviating jet lag.) Clinically, measurement of physiological variables depends on the time of day, and the timing of therapies can improve their effectiveness or reduce side effects. A final chapter places both chronobiology and chronobiological research into their broader philosophical, historical, and social contexts.
Though no background is assumed and basic science is explained — there's even an explanation of the structure of DNA — Rhythms of Life may have too much detail for those primarily attracted by the practical aspects which are played up in the back-cover blurb. For anyone curious about the actual science behind the daily rhythms of life, however, it's an excellent book.