Fig 4. Diagram of four-state Markov model accounting for the wk-sleep dynamics across dark/light cycle.

Circular arrows correspond to the probability of maintaining a state (i.e., the time spent in the corresponding state or bout duration), and straight arrows to transitions between states; arrows thickness are proportional to the corresponding probabilities (from Table 1), and the dark and light periods are represented in black and grey. The sleep-wake model of four states comprises of two wk states (with spectral differences, see text): a long-wk (wkl) and a brief-wk (wkb); and of nrem and rem sleep states. States of wkl and nrem were more stable than rem and wkb, while state transitions wkb to nrem and rem to nrem were the most probable. Circadian modulation increased the stability of wkl mainly by reducing the transitions from wkl to nrem during dark active period (*, p < 0.05; and **, p < 0.01).