What is it about the nature of slow-wave sleep that makes it difficult to awaken the sleeper?

What happens during slow-wave sleep?

If you are woken during SWS, you often feel disoriented and groggy. After falling asleep, your body soon transitions from light sleep to slow wave sleep (then later to REM sleep). Your first period of SWS usually lasts 45-90 minutes. As the night goes on, episodes of SWS become shorter in the sleep cycles that follow.

What characterizes slow-wave sleep?

Slow wave sleep (SWS) is that portion of non–rapid eye movement (NREM) sleep characterized by high-amplitude, low-frequency brain waves.