This thesis aims to investigate the cognitive changes that follow the transition from wakefulness to sleep. All studies presented have been driven by the hypothesis that consciousness is selectively required for some cognitive processes and not others. As such, fluctuations in alertness are hypothesised expected to produce a larger effect in those processes that are more reliant on consciousness. In chapter 2 we begin by investigating the effect of alertness on conflict monitoring. Conflict monitoring is the environmental scanning process by which the brain detects conflicting information and identifies the need for top-down behavioural control. Using a combined Go/NoGo and Go-left/Go-right paradigm, we investigated effects of alertness on reaction times and responsiveness. We revealed an interaction between the task requirement to monitor conflict (Go/NoGo) and the level of alertness. Crucially, the difference between both tasks was only observed in the later stage of drowsiness. This suggests that conflict monitoring is more susceptible to reductions in alertness. Our results from Chapter 2 also provide further evidence for the ecological validity of the Hori scale as a way to classify of the sleep onset period into functionally meaningful sub-stages. However, manual scoring of trials is an arduous process subject to interrater variability. In order to overcome these limitations, we developed an automated algorithm to classify trials into 3 alertness categories using Hori stage principles. This microstaging method was published in the journal Neuroimage (Jahannathan, Ezquerro-Nassar, et al., 2018) and is included in its published form. The method in Chapter 3 was then employed in Chapters 4 and 5, where we investigated effects of alertness and sleep deprivation in an auditory Simon task. This research was published in Journal of Neuroscience (Canales-Johnson, et al., 2020) and the manuscript is included in this thesis. Participants attended two sessions in the lab and performed an auditory Simon task, after a night of normal sleep or a night of partial sleep deprivation. We hypothesised that conflict detection processes necessary for an immediate conflict effect would be preserved under drowsiness, in line with studies showing residual processing for local, short stimuli during early sleep. Similarly, we predicted that sleep deprivation would not impair conflict detection. Furthermore, we investigated the capacity to modify behavioural control using information from the previous trial via top-down mechanisms, known as conflict adaptation. The process that mediates the conflict adaptation effect is hypothesised to require consciousness. Therefore, we hypothesised that trial-by-trial conflict adaptation would be impaired during drowsiness and sleep deprivation. In Chapter 4, we confirmed our hypothesis that the conflict detection is preserved under drowsiness, as indicated by slower reaction times for incongruent stimuli. However, we revealed an interaction between alertness, previous trial congruency and current trial congruency, suggesting a deleterious effect of drowsiness on conflict adaptation. At odds with our hypothesis, sleep deprivation was found to have a main effect on reaction times but it did not interact with any of the variables. In Chapter 5, we found an increase in theta power associated to incongruent trials, a classic neural marker of cognitive control. This effect was observed during wakefulness but not during drowsiness, suggestive of potential alternative mechanisms underlying cognitive control under reduced alertness. Overall, the findings of this thesis suggest that alertness has a different bearing depending on the cognitive process required. This has wider implications for the functional role of consciousness and suggests that sleep onset period is a useful framework to evaluate theories of consciousness.