A reference sleep slow wave activity metric determination system comprising: one or more stimulators configured to provide stimulation to a subject; one or more sensors configured to generate output signals conveying information related to sleep slow wave activity in the subject and stimulation provided to the subject during sleep sessions; and one or more hardware processors operatively communicating with the one or more stimulators and the one or more sensors, the one or more hardware processors configured by machine-readable instructions to: control the one or more stimulators to provide stimulation to the subject during the sleep sessions according to a predetermined therapy regime; determine a sleep session sleep slow wave activity metric and a sleep session stimulation metric for a given sleep session based on the output signals; compare the sleep session sleep slow wave activity metric to a reference sleep slow wave activity metric determined based on prior sleep sessions where stimulation was provided to the subject; and determine an updated reference sleep slow wave activity metric based on the comparison and the sleep session stimulation metric, wherein the one or more hardware processors are configured such that the sleep session sleep slow wave activity metric is compared to the reference sleep slow wave activity metric to determine sleep slow wave activity enhancement in the subject for the given sleep session, wherein the one or more hardware processors are configured such that the comparison comprises determining a difference between the sleep session sleep slow wave activity metric and the reference sleep slow wave activity metric and normalizing the difference by the reference sleep slow wave activity metric. one or more stimulators configured to provide stimulation to a subject; one or more sensors configured to generate output signals conveying information related to sleep slow wave activity in the subject and stimulation provided to the subject during sleep sessions; and one or more hardware processors operatively communicating with the one or more stimulators and the one or more sensors, the one or more hardware processors configured by machine-readable instructions to: control the one or more stimulators to provide stimulation to the subject during the sleep sessions according to a predetermined therapy regime; determine a sleep session sleep slow wave activity metric and a sleep session stimulation metric for a given sleep session based on the output signals; compare the sleep session sleep slow wave activity metric to a reference sleep slow wave activity metric determined based on prior sleep sessions where stimulation was provided to the subject; and determine an updated reference sleep slow wave activity metric based on the comparison and the sleep session stimulation metric, control the one or more stimulators to provide stimulation to the subject during the sleep sessions according to a predetermined therapy regime; determine a sleep session sleep slow wave activity metric and a sleep session stimulation metric for a given sleep session based on the output signals; compare the sleep session sleep slow wave activity metric to a reference sleep slow wave activity metric determined based on prior sleep sessions where stimulation was provided to the subject; and determine an updated reference sleep slow wave activity metric based on the comparison and the sleep session stimulation metric, wherein the one or more hardware processors are configured such that the sleep session sleep slow wave activity metric is compared to the reference sleep slow wave activity metric to determine sleep slow wave activity enhancement in the subject for the given sleep session, wherein the one or more hardware processors are configured such that the comparison comprises determining a difference between the sleep session sleep slow wave activity metric and the reference sleep slow wave activity metric and normalizing the difference by the reference sleep slow wave activity metric.