Internal Time. Chronotypes, Social Jet Lag, and Why You're So Tired.

Early birds and night owls are born, not made. Sleep patterns may be the most obvious manifestation of the highly individualized biological clocks we inherit, but these clocks also regulate bodily functions from digestion to hormone levels to cognition. Living at odds with our internal timepieces, Till Roenneberg shows, can make us chronically sleep deprived and more likely to smoke, gain weight, feel depressed, fall ill, and fail geometry. By understanding and respecting our internal time, we can live better.

Internal Time combines storytelling with accessible science tutorials to explain how our internal clocks work-for example, why morning classes are so unpopular and why "lazy" adolescents are wise to avoid them. We learn why the constant twilight of our largely indoor lives makes us dependent on alarm clocks and tired, and why social demands and work schedules lead to a social jet lag that compromises our daily functioning.

Many of the factors that make us early or late "chronotypes" are beyond our control, but that doesn't make us powerless. Roenneberg recommends that the best way to sync our internal time with our external environment and feel better is to get more sunlight. Such simple steps as cycling to work and eating breakfast outside may be the tickets to a good night's sleep, better overall health, and less grouchiness in the morning.

Till Roenneberg is Professor at the Institute of Medical Psychology at the Ludwig-Maximilians University, Munich.