If you are looking for a book to read over break, the Randy Gardner book is to be recommended. In particular I found this quote to be right on the mark:
“Use it or lose it” is a truism in many walks of life, and it certainly applies to horn playing. Mastering the Horn’s Low Register is narrowly targeted to excellence in low register performance. However, if while working on these specific techniques you neglect high register work, for even a day or two, you will notice increased difficulty in that range. You must practice in all registers every day. Horn students are often surprised to learn that many professional hornists in second and fourth seats spend a large percentage of their practice time in the high register, and that many hornists in first and third horn seats spend a large percentage of their practice time in the low register.
Randy Gardner, Mastering the Horn’s Low Register, pp. 38-39.