Hi Mickey's Dreamers!
We stayed at The Sequoia this week and it appears that breakfast is served from 7.00 onwards.
However, that assumes that you can get a ticket for that sitting, as when we arrived at 4.00 pm on a quiet Tuesday in December, reception told us that the only sitting with availability was 7.45 for the Weds and the Friday, sort of take it or leave it.
It suited us fine (although the mornings were very dark), but may not suit all.
My tip - check in as early as you can to get the full choice, but 7.00 is the answer to your question I reckon. One other thing - early breakfast is gonna give you a lot of time to kill in the winter when parks open at 10.00 during the week.....