DF = data.frame(
    shrew=as.factor( 1:6 ),
    lsws=c( 14.1, 26.0, 20.9, 19.0, 26.1, 20.5 ),
    dsws=c( 11.7, 21.1, 19.7, 18.2, 23.2, 20.7 ),
    rem=c( 15.7, 21.5, 18.3, 17.0, 22.5, 18.9 )
)

library(reshape2)
DF_melt = melt(DF, id.vars=c('shrew'), value.name='heart_rate', variable.name='sleep_stage')