Use it deliberately
Save unsaved work and make sure no task needs the Mac to remain awake before running the command. The local pmset manual defines sleepnow as an immediate system sleep action. It is different from displaysleepnow, which turns off the display, and from changing an idle sleep timer. Run the command from a local Terminal only when an immediate sleep is the intended result.
A controlled sleep can help establish a baseline. Note the start time, power source, connected accessories, and network state, then leave the Mac undisturbed long enough for the symptom you are testing. After waking it normally, inspect the event history instead of assuming that the request itself explains what happened later.
pmset sleepnowIf the Mac does not stay asleep
Use pmset -g log to review sleep and wake entries and pmset -g assertions to inspect current power assertions. Apple also recommends checking sleep settings, Wake for network access, sharing, background activity, and connected storage or devices. Disconnect a suspicious peripheral one at a time only if doing so is safe for your work.
Do not repeat sleepnow in a loop, kill system processes, or make several preference changes at once. Those steps make it harder to compare results and can disrupt active work. Change one relevant setting only after you have identified a plausible cause, then run a later follow-up test.
pmset -g log
pmset -g assertionsWhat WattGuard records
WattGuard organizes the resulting sleep session, including sleep and wake events, charge changes, wake-reason clues and completed sleep blockers. In Full, Sleep Shield uses an installed helper; manual power settings use a macOS administrator prompt. These system power controls are not available in Store. Review the recorded session to see what actually happened after your test.