The headache was getting worse and worse as the night came closer.
"夜にかけて頭痛がひどくなっていった"
頭痛 a headache ここではその頭痛ということで、the headache
ひどくなっていった get worse and worse
夜にかけて 夜になるにつれて
as the night comes closer, as the night gets closer
The headache was getting worse and worse as the night came closer.
日本 
