A diagnostic sleep study in the United States usually means spending a night in a laboratory wired to sensors. The format follows from what has to be measured and when.
Sleep stages are defined by signals, not by behavior
Sleep is scored from electrical activity in the brain, eye movement and muscle tone. The stages are defined by patterns in those recordings rather than by how a sleeper looks.
Distinguishing light sleep from deep sleep, and either from dreaming sleep, requires all three channels together. A single measure cannot separate them reliably.
Because the patterns only occur during sleep, they cannot be captured in a daytime appointment. The recording has to happen while the person is actually asleep.
Several body systems are recorded at once
A full study also records airflow, breathing effort at the chest and abdomen, blood oxygen, heart rhythm, leg movement and body position.
The diagnostic value lies in the alignment between channels. A drop in oxygen matters differently depending on whether effort continued and which stage the sleeper was in.
That is why the study is a single synchronized recording rather than a set of separate measurements taken in sequence.
A technologist has to be present
Sensors detach, a sleeper turns over, and a channel can fail silently. A technologist monitors the signals live and repairs problems without fully waking the patient.
For some conditions the same night includes a treatment trial, with airway pressure introduced and adjusted in real time once events are observed.
Doing that requires someone watching the recording as it happens, which is the other reason the study takes place in a staffed facility.
The lab environment distorts what it measures
Sleeping in an unfamiliar room with sensors attached is not typical sleep. Clinicians expect a first-night effect, with lighter and more fragmented sleep than usual.
Scoring accounts for this, and the conditions being investigated generally still appear, since breathing events and limb movements are not suppressed by unfamiliarity.
Where results conflict with the reported history, a repeat study or a different approach may be used, which is a clinical judgment rather than a standard step.
Home testing covers a narrower question
Home sleep apnea tests record fewer channels, typically airflow, effort, oxygen and pulse, without the brain activity needed to score stages.
That is adequate for suspected obstructive sleep apnea in a person without complicating conditions, and it is cheaper and closer to normal sleep.
Which test is appropriate is a decision for a physician, since the choice depends on what is suspected and on other conditions the patient has.