noun
first thing after waking up; upon rising
Refers to the time or action immediately after getting out of bed. Often used adverbially with に.
起きがけにコップ一杯の水を飲むようにしている。
I make it a habit to drink a glass of water first thing after waking up.
起きがけのストレッチは体にいい。
Stretching right after getting up is good for the body.
Compound of 起き (oki, stem of 起きる 'to get up') + がけ (gake, suffix meaning 'at the time of doing something'). The suffix がけ is related to 掛け (kake), indicating the start or onset of an action.