insomniac
Concise Oxford English Dictionary © 2008 Oxford University Press:
insomnia/ɪnˈsɒmniə/
▶noun habitual sleeplessness.
– derivatives
insomniac noun & adjective.
insomniac noun & adjective.
– origin C17: from L., from insomnis ‘sleepless’.
'insomniac' also found in these Oxford entries:

