Thank you to everyone for the interesting visualizations, it helped me learn a lot. I realized that my original idea for a 'clock-face' creates a confusing and misleading visual when you have activities that span midnight for two nights in a row. I switched instead to a semicircular or fan-like shape instead.

A segment graph also works well in terms of being able to most easily compare days so that you can detect patterns in when the baby is most commonly awake/asleep, while the spiral track most elegantly deals with the issue of activities crossing midnight.

library(ggplot2)
library(lubridate)
library(tidyverse)
library(googlesheets4) #https://googlesheets4.tidyverse.org/
library(aptheme)
library(ggpubr) # allows us to use ggarrange
library(glue)

#Load in sample data from a public Google Sheet 
gs4_deauth() 
url = # insert url here
df = read_sheet(url) 

# Don't want to actually share this Google Sheet so here's reproducible data for seven days:

df = structure(list(date = structure(c(1756512000, 1756512000, 1756512000, 
1756512000, 1756512000, 1756512000, 1756512000, 1756512000, 1756512000, 
1756598400, 1756598400, 1756598400, 1756598400, 1756598400, 1756598400, 
1756598400, 1756598400, 1756684800, 1756684800, 1756684800, 1756684800, 
1756684800, 1756684800, 1756684800, 1756684800, 1756684800, 1756684800, 
1756771200, 1756771200, 1756771200, 1756771200, 1756771200, 1756771200, 
1756771200, 1756771200, 1756771200, 1756857600, 1756857600, 1756857600, 
1756857600, 1756857600, 1756857600, 1756857600, 1756857600, 1756857600, 
1756857600, 1756857600, 1756944000, 1756944000, 1756944000, 1756944000, 
1756944000, 1756944000, 1756944000, 1756944000, 1756944000, 1757030400, 
1757030400, 1757030400, 1757030400, 1757030400, 1757030400, 1757030400, 
1757030400, 1757030400), class = c("POSIXct", "POSIXt"), tzone = "UTC"), 
    activity = c("sleep", "feed", "sleep", "feed", "sleep", "feed", 
    "sleep", "feed", "sleep", "sleep", "sleep", "feed", "sleep", 
    "feed", "sleep", "feed", "sleep", "sleep", "feed", "feed", 
    "sleep", "feed", "sleep", "feed", "sleep", "feed", "sleep", 
    "sleep", "feed", "sleep", "feed", "sleep", "feed", "sleep", 
    "feed", "sleep", "sleep", "feed", "sleep", "feed", "sleep", 
    "feed", "sleep", "feed", "sleep", "feed", "sleep", "feed", 
    "sleep", "feed", "sleep", "feed", "sleep", "feed", "sleep", 
    "feed", "sleep", "feed", "sleep", "feed", "sleep", "feed", 
    "sleep", "feed", "sleep"), start_time = structure(c(-2209158000, 
    -2209152600, -2209150800, -2209139100, -2209132800, -2209122000, 
    -2209113600, -2209096800, -2209089600, -2209152600, -2209145400, 
    -2209139100, -2209132200, -2209122000, -2209114800, -2209100400, 
    -2209095000, -2209150800, -2209148100, -2209146600, -2209132800, 
    -2209123800, -2209122000, -2209113000, -2209101360, -2209096800, 
    -2209093200, -2209158900, -2209153200, -2209150800, -2209139400, 
    -2209134600, -2209122900, -2209115700, -2209097400, -2209089600, 
    -2209157400, -2209151400, -2209149000, -2209138800, -2209132800, 
    -2209120200, -2209114200, -2209096800, -2209091400, -2209083900, 
    -2209082400, -2209153800, -2209150800, -2209140000, -2209135500, 
    -2209123800, -2209114800, -2209097700, -2209089600, -2209080600, 
    -2209157100, -2209150800, -2209149000, -2209138200, -2209132800, 
    -2209122600, -2209113000, -2209096200, -2209089600), class = c("POSIXct", 
    "POSIXt"), tzone = "UTC"), end_time = structure(c(-2209152600, 
    -2209152000, -2209140000, -2209138200, -2209127400, -2209120560, 
    -2209105200, -2209093200, -2209154400, -2209147200, -2209140000, 
    -2209138200, -2209131600, -2209120200, -2209107600, -2209096800, 
    -2209158000, -2209148100, -2209147200, -2209146000, -2209125600, 
    -2209122900, -2209113900, -2209110900, -2209098360, -2209095900, 
    -2209079700, -2209153500, -2209152300, -2209140900, -2209138200, 
    -2209129200, -2209121700, -2209107000, -2209094100, -2209161000, 
    -2209152000, -2209150800, -2209140000, -2209137900, -2209128000, 
    -2209119000, -2209105800, -2209093200, -2209084200, -2209083600, 
    -2209154400, -2209152900, -2209141800, -2209138800, -2209130100, 
    -2209122600, -2209107600, -2209094400, -2209082400, -2209078800, 
    -2209151700, -2209150200, -2209139400, -2209137000, -2209128300, 
    -2209120800, -2209106400, -2209093200, -2209077000), class = c("POSIXct", 
    "POSIXt"), tzone = "UTC")), row.names = c(NA, -65L), class = c("tbl_df", 
"tbl", "data.frame"))

# Adjust so we have the correct days (rather than Google's default of December 30, 1899)
# Midnight flag marks activities that go from one day to the next 

df = df %>%
  mutate(midnight_flag = if_else(end_time - start_time < 0,
                                 TRUE,
                                 FALSE)) %>% 
  mutate(start_time = update(start_time, year = year(date), month = month(date), day = day(date)),
         end_time = update(end_time, year = year(date), month = month(date), day = day(date))) %>% 
  mutate(end_time = if_else(midnight_flag,
                            end_time + days(1),
                            end_time)) %>% 
  mutate(activity = str_to_title(activity))

# Split up days that cross midnight into two rows
df = df %>%
  split(seq_len(nrow(.))) %>% # Split dataframe by row into a list of tibbles
  map_dfr(function(row) { # function to bind rows together into a dataframe again
    if (row$midnight_flag) {
      midnight = ceiling_date(row$start_time, unit = "day")
      just_before_midnight = update(row$date, hour = 23, minute = 59, second = 59)
            tibble( # Creates tibble with two rows
              activity = row$activity,
              date = c(row$date, midnight),
              start_time = c(row$start_time, midnight),
              end_time = c(just_before_midnight, row$end_time)
            )
    } else {
      row %>% select(-midnight_flag)
    }
  }) %>% mutate(date = as_date(date))

# Visualize as semicircles
semicircle_graph = function(starting_date) {
  midnight_start <- ymd_hms(paste(starting_date, "00:00:00"))
  midnight_end <- ymd_hms(paste(starting_date + days(1), "00:00:00"))
  weekday = strftime(starting_date, '%A')
  
  df %>% 
    filter(date == starting_date) %>% 
    mutate(weekday = strftime(date, '%A')) %>% 
    ggplot(aes(xmin = start_time, xmax = end_time,
               ymin = 0, ymax = 1, fill = activity)) +
    geom_rect() +
    scale_x_datetime(limits = c(midnight_start, midnight_end),
                     date_labels = "%l %p",
                     expand = c(0,0), # Comment this out and the shape will be more like a fan
                     breaks = seq(midnight_start, midnight_end, by = "4 hours")) +
    theme_minimal(base_family = "Chalkboard") +
    ggtitle(glue("{weekday}, {starting_date}")) + 
    ggtitle(paste0(weekday, ", ", starting_date)) +
    theme(axis.text.y = element_blank(),
          axis.ticks.y = element_blank(),
          plot.title = element_text(hjust = 0.5, face=1),  
          legend.position = "bottom",
          legend.title = element_blank()) + 
    coord_radial(start = -0.5 * pi, end = 0.5 * pi)  
}

# Plot each day and save as a list
plots = map(unique(df$date), semicircle_graph)

ggarrange(plotlist = plots,
          common.legend = TRUE,
          legend = "bottom",
          nrow = 2, ncol = 4)