Sensitivity is how much voltage is needed to move the EEG trace a certain amount vertically. In a sleep study you want to clearly see both large waves and smaller, transient features like sleep spindles and K-complexes without clipping or losing detail. A middle-ground setting of seven microvolts per millimeter is commonly used because it provides enough vertical resolution to display typical EEG amplitudes and those transient features, while keeping the trace within the recorder’s dynamic range. If the gain is set too high (more sensitive, like one or three µV/mm), large waves or artifacts can saturate the trace and obscure interpretation. If the gain is too low (less sensitive, like fifteen µV/mm), small but important features become hard to see, making accurate sleep staging more difficult.

The main idea is confirming that the system correctly recognizes and responds to the signals from the electrodes when prompted. Verifying electrode response commands means you’re checking that the electrodes produce the expected responses to calibration prompts, ensuring the device properly interprets those signals and that the leads are functioning and contacting correctly. This is essentially the same concept as the stem’s statement, just stated in the term that directly describes the signals from the electrodes. Why the other options don’t fit: keyboard calibration would relate to input devices, not to electrode signals. Bed calibration targets the physical bed setup, not the electrode system. ECG polarity verification focuses on the directionality of the ECG leads, not on whether the electrode signals themselves respond to calibration commands.

Salt bridges form when a conductive path links two electrode sites through moisture or electrolyte, creating an artifact that can mimic or distort the signal. If the skin prep area is too large, conductive gel or electrolyte can spread and reach neighboring electrodes, bridging them. Tape or gauze from different locations touching can also create a conductive bridge between contacts. An incorrect measuring/setup can place electrodes too closely or allow materials to contact them improperly, further enabling a bridge. Because each of these scenarios can produce the conductive connection that defines a salt bridge, all of these factors together explain why this artifact occurs. Prevent by keeping prep areas small, preventing cross-contact with tape/gauze, and ensuring proper electrode placement and impedance checks.

Central sleep apnea results from a disruption in the brain’s ability to send the proper signals to breathe. It’s driven by problems with the central respiratory control centers or their regulation, not by the anatomy of the airway. That’s why congestive heart failure can contribute—Cheyne-Stokes-type patterns reflect instability in central drive and circulation. Brain stem injury can directly damage the medullary centers that control breathing, and neurological diseases affecting central respiratory control can also cause central apneas. Obesity, on the other hand, is a major risk factor for obstructive sleep apnea, where the airway collapses during sleep due to excess soft tissue and reduced airway caliber. It’s also linked to obesity hypoventilation syndrome, which is about inadequate ventilation rather than failure of central drive. Therefore obesity is not a typical cause of central sleep apnea.

If you haven’t fallen asleep after about 20 minutes, the best move is to get out of bed and do something relaxing until you feel sleepy again. This approach helps break the pattern of wakefulness being tied to the bed, reducing anxiety about not sleeping and preserving the association between your bed and sleep. It keeps your overall sleep pressure from being harmed and makes it more likely you’ll fall asleep quickly when you return to bed. Staying still in bed can teach the brain that the bed is for lying awake, which can lengthen wakefulness. Turning on the TV, especially at high volume, is stimulating and bright, which wakes you up rather than relaxing you. Taking a long nap right away reduces the body's need for sleep at night by lowering sleep pressure, making it harder to sleep when you want to later.