Participants will receive active anode HD-tDCS during slow-wave sleep (SWS) of non-rapid eye movement sleep (NREM) at afternoon nap. An anode stimulation electrode will be placed in F3 (10/20 system) which intended to target the left DLPFC, and four cathodes will be placed at FCz, AFz, F7, and C5 (10/20 system)

Participants will receive afternoon nap with sham stimulation. A 30-s rising current until 2 mA is reached, with a 30-s decline immediately at the beginning and end of the stimulation section

Participants will receive active anode HD-tDCS during wakefulness. They will receive HD-tDCS stimulation for 25 minutes while awake, then spent free time in the sleep monitoring room, remaining awake throughout the napping period

HD-tDCS is described as a non-invasive form of brain stimulation that uses a low-intensity, direct current applied directly to the head through scalp electrodes

In the face-cued word association test, participants viewed 20 grayscale photographs of faces on a computer screen, each shown for 3 seconds. Each photograph was paired with a distinct word, which participants read aloud when shown. Participants were instructed to memorize the word linked to each face. One minute later, participants viewed the same 20 faces in a new random order. They were then asked to recall the associated words. Each face was scored as correct or incorrect, and the total number of correct responses formed the AM score.

There are two parts to the test process: learning phase and delayed recall phase. During the learning phase, subjects were asked to listen to a list of words and recall them. During the delayed recall phase, subjects were asked to recall the list of previously learned words again after a period of time. Scoring usually includes recording the number of correct memories the subject has during the learning phase and delayed recall phase, as well as the types of possible errors (such as forgetting, confusion, etc.). Depending on the subject's performance, learning and memory abilities can be assessed. The normal ranges of immediate memory, delayed memory and recognition memory were 10, 8 and 10 respectively. Higher scores mean a better outcome.