Using ifelse() with an interval as conditional expression in R

My dataset looks like this:

> head(GLM_df)
# A tibble: 6 x 9
# Groups:   hour [6]
   hour Feeding Foraging Standing ID    Area     Feeding_Foraging Feeding_Standing Standing_Foraging
  <int>   <dbl>    <dbl>    <dbl> <chr> <chr>               <dbl>            <dbl>             <dbl>
1     0    3.5      23.3     1    41361 Seronera             26.8             4.5               24.3
2     1    2.71     24.2     1    41361 Seronera             26.9             3.71              25.2
3     2    2.5      24.3     2    41361 Seronera             26.8             4.5               26.3
4     3    6.88     18.7     3.89 41361 Seronera             25.5            10.8               22.6
5     4    7.5      17.6     3.67 41361 Seronera             25.1            11.2               21.3
6     5    7.22     19.6     2.45 41361 Seronera             26.8             9.67              22.1

And I simply would like to add a column Day_Night, which will have Day for a value in GLM_df$hour between 6 and 18 or will have Night otherwise.

I tried using the expression below but I'm getting the following error:

> GLM_df$Day_Night<-ifelse(GLM_df$hour==(6:18),Day,Night)
Error in ifelse(GLM_df$hour == (6:18), Day, Night) : 
  object 'Day' not found
In addition: Warning message:
In GLM_df$hour == (6:18) :
  longer object length is not a multiple of shorter object length

Any help is appreciated!