Figure 1.

The complex association between intensive care unit (ICU) sleep and outcomes. Key factors that may influence ICU sleep are shown in red. Notoriously, all are also known contributors to mortality, generating a competing risk scenario (death competes with poor sleep). This becomes even more cumbersome as further follow-up evaluations of sleep are performed: only survivors can be referred to post-ICU sleep studies. The arrow in green represents the assumption that poor sleep in the ICU may cause post-ICU syndrome. No simple adjustment can remove all open pathways for causal inference using a simple regression or competing risk model.