In this case, it's better if researchers do not know which treatment the
participants received. This is called a double blind experiment, because neither
the participants nor the researchers studying them know which treatment they
received. Single blind is when only the participants are unaware of their
treatment condition. And the reason this experiment is double blind is because
if the researcher knows which treatment each person received, that may impact
their judgements on how well the active pill works versus the Placebo. It may
subconsciously cause the researchers to think that the people who took the
active pill are sleeping better, and this might consciously or unconsciously
alter their measurements.