Narrative:

Had been issued a block altitude clearance and been changing altitude as needed within that block (related to turbulence) for several hours. Requested deviation around weather. ATC granted 50 NM. During deviation; descended from FL370 to FL360 due to turbulence. When ATC requested that we confirm altitude; signaled that something was amiss. Reread the deviation message...both of us had previously missed the cancellation of our block clearance- we both thought we still had one. Climbed back to FL370 and flight continued normally to destination.there were several [causes]; speaking only for myself. This was my first flight after nearly X months off due to covid. I still feel confident; but out of practice. [I] was in seat 5+ hours when event occurred; energy was waning a bit. New to the X type aircraft fleet and just consolidated prior to extended time off. Had only crossed ocean X twice and unfamiliar with 'the norms' (didn't know that ATC tends to cancel block clearances in conjunction with another instruction/clearance over ocean X.) expected that an altitude clearance/cancellation would be communicated on its own; not included with weather deviation. Due to aforementioned factors; regrettably didn't register the block cancellation. Captain evidently didn't either.I admit and sincerely apologize for the error; however; altitude clearances are critical to our safety and should be given separately from any other clearance or instruction. One cpdlc message sounds (one chime in the flight deck) - I believe it's human to expect the message to address one thing; not two. It's easier to miss the 2nd item if clearances are combined; especially after many hours sitting with low activity. My suggestion is to send the weather deviation clearance as one message (one chime) and the block altitude cancellation as a separate message (2nd chime.)

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Original NASA ASRS Text

Title: Air carrier First Officer reported an altitude deviation after descending to avoid turbulence. The crew thought they were still cleared to a block altitude; but realized they were not after ATC asked them to confirm their altitude.

Narrative: Had been issued a block altitude clearance and been changing altitude as needed within that block (related to turbulence) for several hours. Requested deviation around weather. ATC granted 50 NM. During deviation; descended from FL370 to FL360 due to turbulence. When ATC requested that we confirm altitude; signaled that something was amiss. Reread the deviation message...both of us had previously missed the cancellation of our block clearance- we both thought we still had one. Climbed back to FL370 and flight continued normally to destination.There were several [causes]; speaking only for myself. This was my first flight after nearly X months off due to COVID. I still feel confident; but out of practice. [I] was in seat 5+ hours when event occurred; energy was waning a bit. New to the X type aircraft fleet and just consolidated prior to extended time off. Had only crossed Ocean X twice and unfamiliar with 'the norms' (Didn't know that ATC tends to cancel block clearances in conjunction with another instruction/clearance over Ocean X.) Expected that an altitude clearance/cancellation would be communicated on its own; not included with weather deviation. Due to aforementioned factors; regrettably didn't register the block cancellation. Captain evidently didn't either.I admit and sincerely apologize for the error; however; altitude clearances are critical to our safety and should be given separately from any other clearance or instruction. One CPDLC message sounds (one chime in the flight deck) - I believe it's human to expect the message to address one thing; not two. It's easier to miss the 2nd item if clearances are combined; especially after many hours sitting with low activity. My suggestion is to send the weather deviation clearance as one message (one chime) and the block altitude cancellation as a separate message (2nd chime.)

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.