Would say about summer because it's refreshing?
ビヤホール（beer hall）ビアガーデン（beer garden）冷や酒（ひやざけ cold sake）冷酒（れいしゅ cold sake）are all 夏の季語．
Physical strength is lost by heat in summer. Also it's hard to sleep well at throughout night. Therefore, 昼寝 is classified in 夏の季語.
However 朝寝(あさね＝a late riser(?)) is 春の季語。Because of the famous Chinese poem named 春暁 written by 孟浩然(Chinese 689-740).
(The original Chinese poem and English translation)
spring or summer.
I also wondered why 風船 is in spring word. Some books explain " Kids play with balloons in spring (during winter, they don't come out from their houses). Others say 風船売り used to be selling balloons in spring time. It is considered that 高浜虚子(たかはまきょし 1874-1959)http://www.big.or.jp/~loupe/links/ehisto/ekyoshi.shtml(Japanese English French are available) took 風船 into 俳句(はいく) as a spring word.
(4) 銀河 天の川
A season where you can clearly see it. Guess an autumn with clear skyes?
Bingo! B) The reason is exactly what you said.