> sleep
> sleep_wide <- data.frame(
  ID=1:10,
  group1=sleep$extra[1:10],
  group2=sleep$extra[11:20]
  )

> head(sleep) 
extra group ID 
1   0.7 1 1 
2  -1.6 1 2 
3  -0.2 1 3 
4  -1.2 1 4 
5  -0.1 1 5 
6   3.4 1 6 

> head(sleep_wide) 
ID group1 group2 
1  1  0.7  1.9 
2  2 -1.6  0.8 
3  3 -0.2  1.1 
4  4 -1.2  0.1 
5  5 -0.1 -0.1 
6  6  3.4  4.4

> t.test(extra ~ group, sleep) 

Welch Two Sample t-test 

data: extra by group 
t = -1.8608, df = 17.776, p-value = 0.07939 
alternative hypothesis: true difference in means between group 1 and group 2 is not equal to 0 
95 percent confidence interval: 
  -3.3654832 0.2054832 
sample estimates: 
mean in group 1 mean in group 2 
  0.75            2.33 

> t.test(extra ~ group, sleep, var.equal=TRUE) 

Two Sample t-test 

data: extra by group 
t = -1.8608, df = 18, p-value = 0.07919 
alternative hypothesis: true difference in means between group 1 and group 2 is not equal to 0 
95 percent confidence interval: 
  -3.363874 0.203874 
sample estimates: mean in group 1 mean in group 2 
                             0.75            2.33 

> t.test(sleep_wide$group1, sleep_wide$group2) 

Welch Two Sample t-test 

data: sleep_wide$group1 and sleep_wide$group2 
t = -1.8608, df = 17.776, p-value = 0.07939 
alternative hypothesis: true difference in means is not equal to 0 
95 percent confidence interval: 
  -3.3654832 0.2054832 
sample estimates: 
mean of x mean of y 
  0.75      2.33 

> t.test(extra ~ group, sleep, paired=TRUE) 

Paired t-test 

data: extra by group 
t = -4.0621, df = 9, p-value = 0.002833 
alternative hypothesis: true mean difference is not equal to 0 
95 percent confidence interval: 
  -2.4598858 -0.7001142 
sample estimates: 
mean difference 
  -1.58 

> t.test(sleep$extra, mu=0) 

One Sample t-test 

data: sleep$extra 
t = 3.413, df = 19, p-value = 0.002918 
alternative hypothesis: true mean is not equal to 0 
95 percent confidence interval: 
  0.5955845 2.4844155 
sample estimates: mean of x 
  1.54