What term best describes a patient who often feels drowsy or has difficulty staying awake?

The term that best describes a patient who often feels drowsy or has difficulty staying awake is "somnolent." This word specifically refers to a state of drowsiness or a tendency to sleep, which captures the essence of the patient's experience accurately. In medical contexts, somnolence indicates that a person is not fully alert and may have impaired responsiveness, which aligns perfectly with the symptoms described in the question.

In contrast, "alert" denotes a state of being awake, attentive, and responsive, which does not fit the description of someone who struggles to stay awake. "Energetic" suggests a person is full of energy and likely actively engaged, which is contrary to the state of feeling drowsy. "Restless" refers to a state of agitation or an inability to relax, which does not inherently describe drowsiness or sleepiness. Thus, somnolent is the most appropriate term for conveying the experience of the patient in question.