Twins are an invaluable resource for comparing the relative effects of environment and genes on behaviour, because identical twins have virtually identical genes, whereas dizygotic twins are no different genetically from other siblings. A large twin study compared the reported sleep and wake patterns of 1190 monozygotic and 2540 dizygotic twins at age 15 months.
The four parameters examined were wake time, night-time sleep duration, day-time nap duration and sleep disturbance. The shared environment was more important than the genetic effect for night-time sleep duration (66% vs. 26%), daytime nap duration (57% vs. 37%) and sleep disturbance (55% vs. 40%). There was no difference in sleep patterns between boys and girls.