1. A method for optimizing sleep of a subject, the method comprising: monitoring at least one physiological characteristic of a subject indicative of a sleep state using signals recorded from the subject's forehead by one or more sensors; using at least one hardware processor of a portable device configured to be worn by the subject, determining a current sleep state of the subject from the at least one monitored physiological characteristic, wherein the current sleep state comprises one of rapid-eye-movement (REM) sleep, non-rapid-eye-movement (NREM) sleep stage one, NREM sleep stage two, and NREM sleep stage three, and determining a desired sleep state for the subject based on one or more rules, the determined current sleep state, and a record of prior sleep states for the subject stored in a memory of the portable device, and wherein the one or more rules are configured to determine a desired sleep state that maintains a balance between REM sleep, NREM sleep stage one, NREM sleep stage two, and NREM sleep stage three and avoids sleep inertia; if the desired sleep state is different than the current sleep state, generating, using at least an eye mask comprising one or more light-emitting elements and one or more heating elements, at least one sensory stimulus to guide the subject toward the desired sleep state; and, if the desired sleep state is the same as the current sleep state, generating, using at least the one or more heating elements of the eye mask, at least one sensory stimulus to cause the subject to remain in the current sleep state.