PLoS One has a paper, A Conserved Behavioral State Barrier Impedes Transitions between Anesthetic-Induced Unconsciousness and Wakefulness: Evidence for Neural Inertia, by Friedman and others here.
One major unanswered question in neuroscience is how the brain transitions between conscious and unconscious states. General anesthetics offer a controllable means to study these transitions. Induction of anesthesia is commonly attributed to drug-induced global modulation of neuronal function, while emergence from anesthesia has been thought to occur passively, paralleling elimination of the anesthetic from its sites in the central nervous system (CNS). If this were true, then CNS anesthetic concentrations on induction and emergence would be indistinguishable. By generating anesthetic dose-response data in both insects and mammals, we demonstrate that the forward and reverse paths through which anesthetic-induced unconsciousness arises and dissipates are not identical. Instead they exhibit hysteresis that is not fully explained by pharmacokinetics as previously thought. Single gene mutations that affect sleep-wake states are shown to collapse or widen anesthetic hysteresis without obvious confounding effects on volatile anesthetic uptake, distribution, or metabolism. We propose a fundamental and biologically conserved concept of neural inertia, a tendency of the CNS to resist behavioral state transitions between conscious and unconscious states. We demonstrate that such a barrier separates wakeful and anesthetized states for multiple anesthetics in both flies and mice, and argue that it contributes to the hysteresis observed when the brain transitions between conscious and unconscious states.
There are a number of pointers in this paper to the nature of consciousness. First is the indication that consciousness is not restricted to humans, or primates, or mammals or even vertebrates. Some of the molecular machinery involved in losing and gaining consciousness probably pre-dated the split between our line and that of the fruit fly. Consciousness, at some level, is likely very old and very general in animals.
Second, the hysteresis between being conscious and being unconscious may be functional. The brain is protected from small fluctuations in the system that might cause repeated fluctuations between consciousness and unconsciousness so that both are relatively stable states. The brain appears to be almost bistable with a fairly clean transition between the two states.
Third, the fall into unconsciousness is quicker (steeper) than the re-establishment of consciousness. This may indicate that consciousness is easier to disrupt than to initiate; it is a more complex state.
This work was done with anesthetics on animals and therefore cannot be applied directly to the sleep-wake cycle in humans. There are parallels though between hysteresis in anesthesia in mice and sleep inertia in humans. It is really hard to wake up, even if it is not Monday!
Friedman EB, Sun Y, Moore JT, Hung H-T, Meng QC, et al. (2010). A Conserved Behavioral State Barrier Impedes Transitions between Anesthetic-Induced Unconsciousness and Wakefulness: Evidence for Neural Inertia.
PLoS ONE 5(7) DOI: 10.1371/journal.pone.0011903